Hastings' Dictionary of the Bible 
Dualism . The belief in, or doctrine of, two ultimate conflicting principles, powers, or tendencies in the universe. Haeckel describes as dualism the distinction between God and the world, and between matter and mind, and opposes to it his monism, which identifies both ( Riddle of the Universe , ch. 1, p. 8). In this sense of the word the Bible teaches dualism. It does distinguish God as Creator from the world as created ( Genesis 1:1 , Isaiah 40:26 , John 1:3 ), and describes God as Spirit in contrast with matter ( John 4:24 ). In man it distinguishes the body taken from the dust, and the spirit given by God ( Genesis 2:7 , Ecclesiastes 12:7 ). This conclusion need not be proved further, as this view is implied in all the teaching of the Bible about God, world, man. But, setting aside this new sense of the term, we must consider whether the Bible gives evidence of dualism in the older sense, as opposing to God any antagonist or hindrance in His creating, preserving, and ruling the world. It is held that dualism in three forms can be traced in the Bible (1) the mythical, (2) the metaphysical, (3) the ethical. Each must be separately examined.
1. Mythical dualism . In the Babylonian cosmology, Marduk , the champion of the upper deities, wages war against Tiamat , who leads the lower deities; at last he slays her, divides her body, and makes part a covering for the heavens to hold back the upper waters. There is little doubt that the account of the Creation in Genesis 1:1-31 reproduces some of the features of this myth, but it is transformed by the monotheism of the author (see Bennett’s Genesis , pp. 67 72). Tiamat appears under the name Rahab in several passages ( Job 9:13 [RV [Note: Revised Version.] ] Job 26:12-13 [see Davidson’s Job , p. 54], Isaiah 51:9; cf. Isaiah 27:1 ‘leviathan the swift serpent,’ ‘leviathan the crooked serpent,’ ‘the dragon that is in the sea’). See Cheyne’s notes on these passages in the Prophecies of Isaiah , i. 158, ii. 31. In illustration of Isaiah 51:9 he quotes the address to Ra in the Egyptian Book of the Dead: ‘Hail! thou who hast cut in pieces the Scorner and strangled the Apophis ’ [ i.e. the evil serpent, Psalms 89:10 , cf. Psalms 74:13-14 ‘the dragons,’ ‘leviathan’]. This name is used as a symbolic name of Egypt ( Psalms 87:4 , Isaiah 30:7 ), probably on account of its position on the Nile, and its hostility to the people of God. The sea is regarded as God’s foe ( Daniel 7:3 ‘four great beasts came up from the sea’; Revelation 13:1 ‘a beast coming up out of the sea,’ Revelation 21:1 ‘the sea is no more,’ that is, the power hostile to God has ceased), a conception in which the myth survives. The influence of the myth is seen only in the poetical language, but not in the religious beliefs of the Holy Scriptures.
2. Metaphysical dualism . Greek thought was dualistic. Anaxagoras assumed hylÃ§ , ‘matter,’ as well as nous , ‘mind,’ as the ultimate principles. Plato does not harmonize the world of ideas and the world of sense. Aristotle begins with matter and form. Neo-Platonism seeks to fill up the gulf between God and the world by a series of emanations. In Gnosticism the plÃ§rÃ´ma and the logos mediate between the essential and the phenomenal existence. St. John ( John 1:1; John 1:14 ) meets this Greek thought of his environment by asserting that Christ is the Word who is with God and is God, and who has become flesh. Against Gnostic heretics St. Paul in Colossians ( Colossians 1:19; Colossians 2:9 ) asserts that the plÃ§rÃ´ma , the fulness of the Godhead, dwells bodily in Christ; to this dualism is opposed the union of Creator and creation, reason and matter in Christ.
From this metaphysical there resulted a practical dualism in Greek thought, between sense and reason. While Aristotle thought that reason might use sense as an artist his material, Neo-Platonism taught that only by an ascetic discipline could reason be emancipated from the bondage of sense; and Stoicism treated sense as a usurper in man’s nature, to be crushed and cast out by reason. Holsten has tried to show that this dualism is involved in St. Paul’s doctrine of the flesh , and Pfleiderer also holds this position. It is held that St. Paul, starting from the common Hebraic notion of flesh ( sarx ), ‘according to which it signifies material substance, which is void indeed of the spirit, but not contrary to it, which is certainly weak and perishable, and so far unclean, but not positively evil,’ advances to the conception of the flesh as ‘an agency opposed to the spirit,’ having ‘an active tendency towards death.’ ‘From the opposition of physically different substances results the dualism of antagonistic moral principles’ (Pfleiderer’s Paulinism , i. 52 ff.). This conclusion is, however, generally challenged with good reason, and cannot be regarded as proved. The question will be more fully discussed in art. Flesh.
3. Ethical dualism . In Persian thought there are opposed to one another, as in conflict with one another, Ormuzd and Ahriman , the personal principles of good and evil. While the OT recognizes the power of sin in the world, yet God’s ultimate causality and sole supremacy are affirmed. In post-exilic Judaism, however, there was a twofold tendency so to assert the transcendence of God that angels must be recognized as mediating between Him and the world, and to preserve His moral perfection by assigning the evil in the world to the agency of evil spirits under the leadership of Satan , the adversary. While these tendencies may be regarded as inherent in the development of Hebrew monotheism, both were doubtless stimulated by the influence of Persian thought with its elaborate angelology and demonology. In the Apocalyptic literature the present world is represented as under Satan’s dominion, and as wrested from him only by a supernatural manifestation of God’s power to establish His Kingdom. This dualism pervades the Apocalypse. In the NT generally the doctrine of the devil current in Judaism is taken over, but the Divine supremacy is never denied, and the Divine victory over all evil is always confidently anticipated. (See artt. Apocalyptic Literature, Devil, Eschatology.)
While in the Bible there are these traces of the threefold dualism, it is never developed; and monotheism is throughout maintained, God’s sole eternity, ultimate causality, and final victory being asserted, while God is distinguished from the world, and in the world a distinction between matter and mind is recognized.
Alfred E. Garvie.
Webster's Dictionary 
(1): ( n.) State of being dual or twofold; a twofold division; any system which is founded on a double principle, or a twofold distinction
(2): ( n.) The theory that each cerebral hemisphere acts independently of the other.
(3): ( n.) The doctrine that all mankind are divided by the arbitrary decree of God, and in his eternal foreknowledge, into two classes, the elect and the reprobate.
(4): ( n.) A system which accepts two gods, or two original principles, one good and the other evil.
(5): ( n.) A view of man as constituted of two original and independent elements, as matter and spirit.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
in philosophy, is that system which explains the phenomena of the universe by assuming two primal principles instead of one (Monism). In theology, Dualism explains evil by assuming two original principles or beings, one good, the other evil. The doctrine of two primal causes, one good and the other evil, constantly warring with each other, lay at the foundation of the system of Zoroaster (q.v.). It was also developed later in Manicheism (q.v.); and among the Sclavonians, who, during the interval between their undisturbed faith in their national mythology and their conversion to Christianity, added to the worship of the good being that of a supremely evil one, viz. Czernebog (the Black God) (London Review, April 1855, page 11). It was in this Sclavonic soil that the Oriental dualism found a congenial home, and from it seems to have originated the dualism of the Cathari and other sects during the Middle Ages. (See Cathari).
Its root is always found in imperfect speculation on the relation of God to the world, and on the origin of evil. It is apt to spring up, also, in the practical sphere, from the sense of personal sin, which seeks relief in a transfer of guilt from the real self the man to something outside of him, e.g. to the physical side of his own nature, or to the general laws of nature.
1. Oriental Dualism . — The Chinese, at a very early period, adopted a dualistic philosophy and theology. The ordinary speech of their philosophers was dualistic, implying two primal essences, "one a power or cause, the other a more passive something on which that power or cause could operate. The former may be styled the ultimate immaterial principle of the universe (Le ); the second, consisting of ethereal matter, is the ultimate material principle (Ke ). The latter, again, is dual ( Yang And Yin ), viz. the paternal and maternal principles in nature. Man is the product of the marriage of the male and female principles in nature. Yang and yin, coexisting as the material ground in which the ultimate principle (Ke) takes effect, enter into the composition of rational as well as of irrational beings. In moral speculation, however, this dualism passed into a sort of pantheism" (Hardwick, Christ and other Masters, part 3, chapter 1).
The Persian Dualism. The Persian system, whether originated by Zoroaster, or, what is more likely modified by him from older doctrines, taught that there is "a supreme Being, all powerful and eternal, from whom have eternally proceeded, by his creative word (Honofer), two principles, Ormuzd and Ahriman; Ormuzd (Oromasdes) being pure and infinite Light, Wisdom, and Perfection, the Creator of every good thing; Ahriman the principle of darkness and evil, opposed to Ormuzd, either originally or in consequence of his fall. To this belief are attached fables respecting the conflicting efforts and creations of these two powers; on the universal dominion ultimately reserved for the good principle, and the return of Ahriman during four periods, each of which is to last three thousand years; on the good and the evil spirits (Amshaspands, Izeds, Ferfers, and Dives), and their differences of sex and rank; on the souls of men (Ferfers), which, created by Ormuzd before their union with the body, have their habitation in the heavens; and which ultimately, according as in this world they have served Ormuzd or Ahriman, pass after death into the dwellings of the blessed, or are precipitated into obscurity: finally, respecting the future resurrection of the bodies of the wicked after the victory of Ormuzd and the restoration of all things" (Tennemann, Manual Hist. of Philosophy, § 71; see also Hardwick, Christ and other Masters, part 3, chapter 3). The Oriental Dualism first sets the Hyle ( Ὕλη , matter) as an original principle over against the divinity. The Eastern philosophers soon found it necessary to run into Pantheism; for, the necessity of unity pressing on them, they found no other way of escape except to make God the soul of the world. But, the gulf between matter and divinity still remaining, they had to fall upon two principles, the material and spiritual; and, not willing to identify the original spiritual principle with matter, darkness, and evil, they fell upon the idea of two antagonistic beings or gods, a good and an evil one, the god of light and the god of darkness, the god of matter and the god of spirit Ahriman the evil principle, and Ormuzd the good.
2. Dualism In The Christian Age . — This Oriental Dualism, carried out into the various departments of nature and mind, and embellished by innumerable beautiful fancies, had a great charm for the imagination of even the primitive Christian mind; and it seemed also to form a certain kind of natural and easy alliance with the doctrines of good and evil, God and Satan, spirit and matter, in the human constitution, as these are unfolded in the Christian revelation, so that this dualistic mode of thinking failed not to insinuate itself largely into the thinking of many in the primitive Church. It has also revealed itself, more or less, in various sects and systems in every period of Christian history, and its false theories have often troubled the mind of the Church in the development and statement of its dogmas. Thus in Gnosticism, and especially in the Docetic phase of it, Dualism enters as a ruling element. The Gnostics found it difficult to explain the existence of the sensible world, and especially the existence of evil, on the direct assumption of one absolutely good Being. Hence they mixed into their theory some elements of the Oriental philosophy. "They thought themselves compelled to combine with the doctrine of emanation that of Dualism, in order, by the commixture of two hostile realms, by the products of two opposite principles, to explain the origin of a world not answering to the divine idea, with all the defects cleaving to it, all the evils it contains" (Neander, Hist. of the Chr. Church, Bohn's ed. 2:14). For the Manichaean Dualism, (See Manicheism); and for that of the Cathari, (See Cathari).
That the ascetic tendencies of the early Christian age were strongly stimulated, if not unconsciously caused by a leaven of Dualism, can hardly be doubted. "A dark instinct of a state of abnormal and dangerous antipathy to God leads the devotee to take vengeance in time upon that part of himself which is outside, and which may be hardly treated, and even tortured, at far less cost than the renewal of the spirit of his mind, and the bringing of his whole inner man back to gravitate towards God instead of turning upon itself. Manes endeavored to unite Christianity and the noblest form of Oriental paganism in his brilliant and elaborately constructed speculative system. The Church repulsed the heresiarch because of his personal pretensions, his rival hierarchy, and his too open importations from the religion of Persia; but it was not the less profoundly modified by the tendencies which it nominally rejected. Monasticism in Syria and Egypt was the direct result of the contact of degenerating Christianity with pagan habits of thought. The idea that abstinence from food was meritorious in itself, the notion of impurity attached to the sexual relation, the growing tendency to look upon marriage as a state less holy than celibacy these were so many triumphs of the invading pagan conception. The errors and extravagances of the ascetic life were especially prevalent in the Eastern Church. Schmid quotes authorities to show that remembrances of Manichaeism were long kept up in Oriental convents, and also that sundry Greek monks, in their solitude, imagined they had constantly to struggle with the devil, whose power they magnified until they put him almost on a rank with God" (London Review, April 1855, page 10; see also Lea, Sacerdotal Celibacy, Phila. 1867, page 42 sq.).
The progress of philosophy and theology in all Christian ages has been a continuous struggle to overcome Dualism, to bring God and the world, the infinite and the finite, heaven and earth, spirit and matter together, and to do this without violence to the essential nature of either, by, on the one hand, confusing them, or, on the other, annihilating one or the other by identification of them. Pantheism, as it has sprung up on the arena of modern theological investigation, has been an earnest, though mistaken effort to overcome Dualism. Much as Pantheism is to be abhorred and dreaded, yet ought its service to be acknowledged in helping philosophy and theology to master Dualism. It has both suggested and stimulated the movement that aims at the creation of a christological theology, and we may also say philosophy, which professes, not without hope of success, to overcome that mischievous Dualism which knows only to negate, and which, in a cowardly manner, has only given up the great fundamental problems. It holds that the great gulf can be, and can only be, bridged by the God man in whose mysterious person all dualismn is overcome the center and perennial source of all life and thought, the principle of all unities and the unity of all principles, the whole of all that is divided, the harmony of all manifoldness and diversity, the center of all science, and the imperial, incarnate Word of all authority and truth, the final rest of all minds, as he is also of all hearts. Hardwick, Christ and other Masters (London. 1863, 2 volumes, 12mo); Dorner, Doctrine of the Person of Christ (see Index); Hagenbach, Hist. of Doctrines, Smith's ed., § 51, 127; Theol. Stud. u. Kritiken (1837), page 357; Lange, Life of Christ (Edinb. 1854, 6 volumes, 8vo), 1:135 sq.; H. Schmid, in Herzog, Real Encykl. 19:432.
The Nuttall Encyclopedia 
he doctrine that there are two opposite and independently existing principles which go to constitute every concrete thing throughout the universe, such as a principle of good and a principle of evil, light and darkness, life and death, spirit and matter, ideal and real, yea and nay, God and Devil, Christ and Antichrist, Ormuzd and Ahriman.