Watson's Biblical & Theological Dictionary 
A Pagan philosopher of the second century, who composed a work against Christianity, in which he so expressly refers to the facts of the Gospels, and to the books of the New Testament, as to have furnished important undesigned testimony to their antiquity and truth.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
a philosopher of the second century, supposed to have been of the Epicurean sect, but inclined towards Platonism. He lived towards the close of the reign of Adrian, and during part of that of M. Aur. Antoninus; and (if Origen be correct) wrote an attack upon the faith and morals of Christians, which he called Λόγος Άληθής , or "A True Discourse," the date of which Lardner supposes to have been about A.D. 176. Our only knowledge of it is derived from Origen's reply to it (Contra Celsum, lib. 8), which, however, gives extracts sufficiently copious to allow a pretty sure judgment of its contents and purpose.
Of the life of Celsus little or nothing is known. Lucian dedicated his life of the magician Alexander to Celsus the Epicurean, and Origen identifies this person with the author of the book against which hea wrote. The spirit of the book is far more Platonic than Epicurean. The arguments for and against the identity of the two persons thus named are stated in Neander, Church History (Torrey's transl. 1:160 sq.); and in Baur, Geschichte der drei ersten Jahrhunderte, p. 371. "Both conclude that the persons were different. The evidence of their oneness is chiefly Origen's conjecture that they were the same person (cont. Celsum, 4:36). The evidence against it is:
(1.) That Lucian's friend attacked magical rites; the Celsus of Origen seems to have believed them.
(2.) That Lucian's friend was probably an Epicurean; the other Celsus a Platonist or Eclectic.
(3.) That the former is praised for his mildness; the latter shows want of moderation. Pressense (Trois Prem. Siecles, vol. 2:105) regards them as the same person" (Farrar, Free Thought, p. 51). It is quite in harmony with the whole spirit of the book, as well as of the Pagan philosophy of the time, to suppose that Celsus is, as Origen supposed, the Epicurean friend of Lucian; and that, in this treatise, he argues on any principles that may serve his purpose. But, whoever Celsus may have been, his writings are very important to Christian apologetics. They "are valuable on account of their admissions of the grand facts and doctrines of the Gospel as preached by the apostles and contained in their writings, by an enemy who lived little more than one hundred and thirty years after the ascension of our Lord. He has nearly eighty quotations from the books of the New Testament, which he not only appeals to as existing, but as universally received by the Christians of that age as credible and divine. He is most minute in his references to the circumstances of the life of Christ and his apostles, which shows that he was well acquainted with them, and that no one denied them.
He everywhere ridicules the idea of our Lord's divinity, contrasting with it that of his poverty, sufferings, and death; which proves not only that the Christians of that early age avowed their belief in the doctrine, but that Celsus himself, though an unbeliever, found it in the documents to which he refers, as the source of his acquaintance with the Christian system" (Buck, s.v.). Moreover, he is the "original representative of a kind of intellect which has presented itself over and over again in the various attacks made on Christianity: wit and acuteness, without earnest purpose or depth of research; a worldly understanding, that glances merely on the surface, and delights in hunting up difficulties and contradictions. His objections against Christianity serve one important end: they present in the clearest manner the opposition between the Christian standing-ground and that of the ancient world; and, in general, the relation which revealed religion will ever be found to hold to the ground assumed by natural reason. Thus many of his objections and strictures became testimonies for the truth" (Neander, 50. 100.).
Lardner (Testimonies, chap. 18; Works, 7:210 sq.) gives full summaries of the book, classed under different heads, especially with reference to the authentication of the books of the N.T., for which these allusions and citations are of special value, as coming from a heathen opponent. A full analysis is also given by Neander, Ch. History, 1:160 sq. (Torrey's transl.), and by Tzschirner, Fall des Heidenthums, 1:320 sq. Pressense, in his Hist. Deuteronomy 1'Elglise des Trois Prem. Si È cles (2d series, 2:140 sq.), attempts ingeniously a reproduction of Celsus's treatise, as gathered from Origen, which Farrar follows (Critical History of Free Thought, lect. 2) in the outline which we here presenit. The references are to the Benedictine edition (Paris, 1733). Celsus intoduces a Jewish rabbi as opposing Christianity from the Hebrew monotheistic point of view. "The rabbi first criticizes the documents of Christianity, and then the facts narrated. He points out difficulties in the Gospel narratives of the genealogy of Christ; utters the most blasphemous calumnies concerning the incarnation; turns the narrative of the infancy into ridicule; imputes our Savior's miracles to magic; attacks his divinity; and concentrates the bitterest raillery on the affecting narrative of our blessed Lord's most holy passion. Each fact of deepening sorrow in that divine tragedy, the betrayal the mental anguish, the sacred agony (2:24), is made the subject of remarks characterized no less by coarseness of taste and unfairness, than to the Christian mind by irreverence. Instead of his heart being touched by the majesty of our Savior's sorrow, Celsus only finds an argument against the divine character of the adorable sufferer (2:16). The wonders accompanying Christ's death are treated as legends (3:38); the resurrection regarded as an invention or an optical delusion (3:59, 55, 57, 78).
"After Celsus has thus made the Jew the means of a ruthless attack on Christianity, he himself directs a similar one against the Jewish religion itself (3, § 1 and elsewhere). He goes to the origin of their history; describes the Jews as having left Egypt in a sedition (3, § 5); as being true types of the Christians in their ancient factiousness (2, § 5); considers Moses to be only on a level with the early Greek legislators (1:17, 18; 1:22); regards Jewish rites like circumcision to be borrowed from Egypt; charges anthropomorphism on Jewish theology (4:71; 6:62), and declines allowing the allegorical interpretation in explanation of it (4:48); examines Jewish prophecy, parallels it with heathen oracles (7:3; 8:45), and claims that the goodness, not the truth of a prophecy, ought to be considered (7:14); points to the ancient idolatry of the Jews as proof that they were not better than other nations (4:22, 23); and to the destruction of Jerusalem as proof that they were not special favorites of heaven. At last he arrives at their idea of creation (4:74; 6:49, etc.), and here reveals the real ground of his antipathy. While he objects to details in the narrative, such as the mention of days before the existence of the sun (6:60), his real hatred is against the idea of the unity of God, and the freedom of Deity in the act of creation. It is the struggle of pantheism against theism.
"When Celsus has thus made use of the Jew to refute Christianity from the Jewish stand-point, and afterwards refuted the Jew from his own, he proceeds to make his own attack on Christianity; in doing which, he first examines the lives of Christians (3), and afterwards the Christian doctrine (5, 6, 7), thus skillfully prejudicing the mind of his readers against the persons before attacking the doctrines. He alludes to the quarrelsomeness shown in the various sects of Christians (3:10), and repeats the calumnious suspicion of disloyalty (3:5, 14), want of patriotism (3, § 55; 8:73), and political uselessness (8:69), and hence defends the public persecution of them (8:69). Filled with the esoteric pride of ancient philosophy, he reproaches the Christians with their carefulness to proselytize the poor (3:44, 50) and to convert the vicious (3:59, 62, 74), thus unconsciously giving a noble testimony to one of the most divine features in our religion, and testifying to the preaching of the doctrine of a Savior for sinners.
"Having thus defamed the Christians, he passes to the examination of the Christian doctrine, in its form, its method, and its substance. His aesthetic sense, ruined with the idolatry of form, and unable to appreciate the thought, regards the Gospels as defective and rude through simplicity (3:55; 8:37). The method of Christian teaching also seems to him to be defective, as lacking philosophy and dialectic, and as denouncing the use of reason (7:9; 1:2; 1:9; 3:39; 6:10). Lastly, he turns to the substance of the dogmas themselves. He distinguishes two elements in them, the one of which, as bearing resemblance to philosophy or to heathen religion, he regards as incontestably true, but denies its originality, and endeavors to derive it from Persia or from Platonism (6:15; 6:22, 58, 62; 5:63; 6:1), resolving, for example, the worship of a human being into the ordinary phenomenon of apotheosis (3:22; 7:28-30). The other class of doctrines which he attacks as false consists of those which relate to creation (4:37; 6:49), the incarnation (4:14; 5:2; 7:36), the fall (4:62,70), redemption (5:14; 7:28, 36; 6:78), man's place in creation (4:74, 76, 23), moral conversions (3:65), and the resurrection of the dead (5:14,15). His point of view for criticizing them is derived from thee fundamental dualism of the Platonic system; the eternal severance of matter and mind, of God and the world; and the reference of good to the region of mind, evil to that of matter. Thus, not content with his former attack on the idea of creation in discussion with the Jew, he returns to the discussion from the philosophical side. His Platonism will not allow him to admit that the absolute God, the first Cause, can have any contact with matter. It leads him also to give importance to the idea of Δαίμρνες , or divine mediators, by which the chasm is filled between the ideal god and the world (7:68; 8:[2-14] 35, 36), not being able otherwise to imagine the action of the pure Ἰδέα of God on a world of matter. Hence he blames Christians for attributing an evil nature to demons, and finds a reasonable interpretation of the heathen worship (8:2). The same dualist theory extinguishes the idea of the incarnation as a degradation of God; and also the doctrine of the fall, inasmuch as psychological deterioration is impossible if the soul be pure, and if evil be a necessary attribute of matter (4:99). With the fall redemption also disappears, because the perfect cannot admit of change; Christ's coming could only be to correct what God already knew, or rectify what ought to have been corrected before (4:3, 7,18). Further, Celsus argues, if Divinity did descend, that it would not assume so lowly a form as Jesus. The same rigorous logic charges on Christianity the undue elevation of man, as well as the abasement of God. Celsus can neither admit man more than the brutes to be the final cause of the universe, nor allow the possibility of man's nearness to God (4:74). His pantheism, destroying the barrier which separates the material from the moral, obliterates the perception of the fact that a single free responsible being may be of more dignity than the universe."
The order in which the objections of Celsus are arranged in Origen's reply to him is different from that above given in some respects, and it is therefore here subjoined: "The first half of book 1 is prefatory (ch. 1-40); the second half, together with book 2, contains the attack by the Jew on Christianity given in lect. 2. The early part of book 3 (1-9) contains Origen's refutation of the Jew. The subsequent parts and remaining books give Origen's refutation of Celsus's own attack on Christianity. First, Celsus attacks the character of Christians in the remainder of book 3. In book 4 he returns to his attack on Judaism, and on the Scriptures of the Old Testament, especially on many of the narratives, either regarding them as false or as borrowed, and objecting to their anthropomorphic character; also objecting to the account of man's place in creation, and of divine interference. In book 5 he continues his attack on the doctrines of both religions, chiefly so far as he considers them to be untrue; and in book 6 so far as he considers them to be borrowed, dragging to light the difference which existed between Judaism and Christianity. In book 7 the subject of prophecy and some other doctrines, as well as the ethics of Christianity, are examined; and in book 8, when the attack on Christianity is mainly over, a defense of paganism is offered by Celsus. Such is the type of a philosophical objector against Christianity a little later than the middle of the second century. We meet here for the first time a remarkable effort of pagan thought, endeavoring to extinguish the new religion; the definite statements of a mind that investigated its claims and rejected it. Most of the objections of Celsus are sophistical, a few are admitted difficulties, but the philosophical class of them will be seen to be the corollary from his general principle before explained."
Literature. — Besides the works already cited, see Cave, History of Literature, 1:96; Pond, in Literary and Theological Review, 4:219, 584; Cudworth, Intellectual System, 2:340 sq. (American edition); Shedd, History of Doctrines, bk. 2, ch. 2; Bindemann, in Illgen's Zeitschrift, etc. 1842, Heft 2; Schaff, Church History, 1, § 60; Jachmann, De Celso, etc. (Regiom 1836, 4to); Hase, Church History, § 51; Fenger, de Celsc Epicurio (1828, 8vo; maintains that Celsus was not a Platonist); Gieseler, Ch. Hist. 1, § 39 (note); Mosheim, Commentaries, cent. 2, § 19 (argues that Celsus was ar Alexandrian Platonist); Baptist Quart. 1868, Jan. and Apr. (See Apologetics); (See Apologies); (See Origen).
The Nuttall Encyclopedia 
A celebrated Roman physician of the age of Augustus, and perhaps later; famed as the author of "De Medicina," a work often referred to, and valuable as one of the sources of our knowledge of the medicine of the ancients.