Webster's Dictionary 
(1): ( a.) Hence, specifically, agency between parties at variance, with a view to reconcile them; entreaty for another; intercession.
(2): ( a.) The act of mediating; action or relation of anything interposed; action as a necessary condition, means, or instrument; interposition; intervention.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
in the Christian sense, is the intervention of Jesus Christ between God and sinners. It implies a condition of alienation and hostility on the part of man towards God, and a corresponding state of disfavor and condemnation in the divine mind with respect to man. Such a mutual relation of dissatisfaction lies at the basis of the whole remedial scheme of salvation, originating in the fall (q.v.), and provided. for in the atonement (q.v.). It is presumed in every form of religion and worship, whether heathen, Jewish, or any other; and has its natural exponents in sacrifice (q v.), the priesthood (q.v.), and ritual (q.v.). In addition to the considerations adduced under the head Mediator (q.v.), there remain certain fundamental aspects of this question which we propose here briefly to discuss. (See Expiation).
1. Man'S Enmity Towards God .-This is a fact too apparent to require detailed proof. Its historical origin is given: in the Bible in the account of Eden, its record is engraven in the whole course of human conduct, and its conclusive attestation is found in the deepest consciousness of man's nature. The sense of guilt and condemnation, to which it inevitably and legitimately gives rise in the human conscience, is a testimony so universal, so profound, and so overwhelming as to call for little if any external corroboration.
2. God'S Displeasure Towards Man .-This is a doctrine ‘ which of necessity results from the preceding one. If God be holy, as the Scriptures represent him, and as the purest forms of faith depict him, he cannot but regard all sin with the utmost abhorrence, and he cannot be supposed to entertain amicable emotions towards those who commit and delight in sin. This feeling in the divine mind, however, must not be regarded as one of vindictiveness or personal hatred. A pure and unselfish being, raised above the petty jealousies and hazards of earth, cannot be conceived as entertaining sentiments of Malice . Such a view of the divine nature is inconsistent with the emphatic statements of Scripture (such as that "God is love," etc.),.with the interest he still Cakes in fallen humanity (" God so Loved the world that he gave his only-begotten Son," etc.), and even with the benevolent provision which he makes in nature for the continuation and comfort of the race. In like manner Christians are forbidden to indulge any malevolence towards their own personal enemies, much more towards their fellow-creatures at large. That view of the Almighty which represents or imagines him as taking any delight in human suffering is characteristic of heathenism, not of Bible truth. (See Love).
Nevertheless the purest ethics, as well as the soundest theology, demands a place in the divine mind for that sense of indignation with moral evil, and that call for its punishment, which are instinctive in ‘ the human breast. In this light. are to be interpreted the many and pointed declarations of the Bible respecting God's anger against sin, and his inexorable determination to inflict vengeance upon its perpetrators. Justice, no less than mercy, is one of the indispensable attributes of a holy deity. The ultimate grounds of this doctrine are not to be sought so much in any considerations of administrative policy or governmental consistency-mere views of expediency and safety-as in the essential contradiction of the divine nature itself to all that is inconsistent with its own character.
3. These premises being settled as the actual relations between the parties, the grand problem arises, How can this mutual disagreement be removed? That the change, if any, must take place in man, is obvious, not only because God is immutable, but because he certainly has not been at fault. The offender alone must make the amends. The Being offended against may indeed propose advances towards reconciliation, as. it belongs to him to lay down the terms of satisfaction, but these cannot involve any concession nor imply any retraction. The standard of righteousness must not be lowered, nor wrong exculpated. The case presents a difficulty in two aspects, neither of which can be overlooked in any scheme proposing its settlement. They relate respectively to the Past and the Future . Two questions therefore arise: 1. How can the sinful acts already committed be properly forgiven? 2. How can their recurrence be most effectively prevented in time to come? These two subordinate problems must be wrought out together, as the omission to solve the latter would render the solution of the former nugatory. The mediation of Jesus Christ exactly meets all the conditions of both these problems. It is spontaneous on the part of God, voluntary on the part of the Mediator, and does not infringe on the freedom of man. It cancels the past debt, takes away the sense of present guilt, and removes the disposition to transgress thereafter. It releases, reconciles, and renews at once. Pardon, peace, purity are its harmonious results. Justification, regeneration, sanctification are its immunities. The first frees from the judicial sentence, the second restores to the heavenly family, and the, third fits for life here and forever. All this is due to the vicarious principle of the atonement. It remains to show more particularly how the substitution of Christ as a victim for man in undergoing the penalty accomplishes these ends successfully and satisfactorily. The transfer of the punishment due to human crimes, as effected in the life and death of our Saviour, is not a mere forensic device, nor simply a diplomatic artifice; it is no stratagem invented to elude justice, nor a pretence set up to screen impunity. If, with regard to its individual objects, it was unconditional and absolute, as Universalism generally on the one hand represents it by extension, and strict Predestinarianism on the other by limitation, it would justly be liable to this charge. But inasmuch as it secures the permanent reformation of the culprit in the very process of amnesty, it is not purely penal, but also prophylactic; it changes the relations of the sinner by converting him into a saint.
(1.) The chief, if not the only difficulty in our conceptions of the method of Christian redemption relates to the justice of substituting an innocent for a guilty person in the expiation of crime. This is, to be sure, an abstract question, but it is a fundamental one. Its determination. however, rests with the Being to be placated, and with the individual submitting to become the victim, rather than with ourselves, the beneficiaries of the arrangement, or with any other intelligences who may be merely spectators. As the compact, in pursuance of which this mediation is effected, was confined to the bosom of the Godhead, we might fairly be excused from attempting its vindication; especially as the Father and the Son, regarded as the contracting parties, are so identified in nature and action that any moral discrepancy or personal disagreement, such as this question implies, is necessarily excluded. Indeed, if they two freely consent, as the plan presupposes, it is hard to see who can have a right to raise a doubt or utter complaint on the subject. Still, to obviate all cavil, it may not be amiss to pursue this point as far as we may without presumption or arrogance.
Instances of a similar but far less extensive vicarious suffering have occurred in human history, and are often pointed to as rare but striking illustrations of this principle. These were applauded at the time of their occurrence, and have been commended ever since by the common voice of mankind without incurring the imputation of unfairness or compromise. If we look into the design of judicial exactions, so far as human legislation and administration enable us to discern it, we find it to be fourfold:
1, the appeasement of the wrath of the injured party;
2, the moral cure of the offending party;
3, the allaying of the sense of wrong in the convictions of the community; and,
4, the deterring of others from similar crimes.
Most laws for earthly retribution have chiefly in view the pecuniary reparation of the wrong, and the protection of society against its recurrence; and in these respects Christ's atonement is as parallel as possible. In cases of capital punishment, with which the present is most analogous, the first two ends of penal infliction are necessarily excluded, by the death of the murdered and the execution of the murderer; so that there remain only the moral influence and the preventive effect upon others as the essential objects to be attained. (See Punishment). But, in the case in hand, these external and disinterested observers can consist only of the angels and inhabitants of other worlds, inasmuch as our own race is wholly included in the culprit himself. Of the moral constitution or even existence of the latter of these two classes of presumed spectators we have absolutely no knowledge, nor any reason to suppose that they could become informed of the transaction. Of the former we know but little more, and that little leads us to the belief that they have already passed their probation, and are therefore incapable of being influenced by example, while the interest which they take in the scene is that of intense satisfaction at its progress and consummation. All objectors are thus removed, and the substitution is ratified by common consent.
We have assumed that man's demurral to this procedure is silenced by the fact of his being himself the convict. Yet a prisoner may be imagined to have a right to protest against another's taking his place as accused or condemned. This, however, he can only be allowed in court to do when he confesses his crime, and demands to bear its penalty in person. Both these privileges, if such they can be called, are reserved to him by the scheme under consideration. Nay, he is required to make confession before he can avail himself of the benefits of Christ's mediation, and that with a sincerity and fulness which admit of no retraction; and he is at last compelled to undergo the penalty himself unless he voluntarily and actively apply for the exemption offered him. These provisions are the saving clauses of the bill of amnesty, and by virtue of them the vicarious redemption receives its final approval.
(2.) Nevertheless the sinner realizes a partial effect of the atonement unconditionally, in the respite from punishment till the close of his earthly career. But for this the whole race had been cut off in embryo at the first transgression. Hence there is an opportunity for the exercise of the remedial or curative as well as preventive influence of that penal retribution, which is temporarily suspended and may be -wholly averted from himself. The only problem here arising is, How can impunity be allowed without encouraging vice? or rather, to state it more radically, How can the criminal go scot-free and yet be reformed? It has of late years only been discovered in families, schools, armies, and diplomacy that pardon is often the best discipline; but God knew long ago the true philosophy of the prevention of crime. The spectacle of another suffering the penalty due to ourselves has been found to be the most effectual softener of the rebel heartland the condition of genuine contrition is the best. safeguard against the abuse of clemency. In this light the scheme of Christian mediation is most abundantly sanctioned by actual experiment, and the Cross becomes the glory of the redeemed. (See Redemption).
(3.) It is not to be imagined, however, that in this vicarious atonement Jesus Christ actually experienced the aggregate amount of suffering due for the sins of every human being. In the first place, this was unnecessary. The object to be attained was not a given amount of penal infliction, whether to placate the Almighty, to reform the offender, or to vindicate the statutes infracted. This is obvious from the foregoing discussion. Had these ends rigidly required an exact balance-sheet of debit and credit on this basis, no substitution or vicarious satisfaction had been admissible at all. The strict terms of the law are," The soul that sinneth, it shall die." The mediation under consideration, was an equivalent, such as met the moral design of the penalty. Nor is it correct to argue that as man incurred infinite guilt by sinning against infinite holiness, so Christ offered an infinite satisfaction by reason of his divine and perfect nature. Neither part of this proposition is tenable. No finite creature is capable of infinite guilt, of even the sum total of all humanity, for it is limited both in its numbers and nature, and so is likewise the sum of its sins. Christ therefore did not need to make an infinite atonement, but only an adequate or commensurate one.
His expiation was sufficient, not because it was made by his divine nature-for that was by hypothesis incapable and incompetent-but because it contained such a degree of merit, in view of its completeness and the exalted character of the offerer, that the divine Being could consistently accept it in Lieu of the actual obedience of the race represented, and thus remit the penalty due them. In the next place, an absolute equality or identity of retribution was impossible in the remedial scheme. The supposition that Jesus endured-whether during his whole lifetime, or in the brief agonies of the garden and the cross-the sum total of the torments that will be and that would have been' experienced by the eternally damned, is simply preposterous. Not only had he no opportunity for this, but he was not capable of it, either physically or spiritually. His bodily pain was such, indeed, as to take his life, but other men have known as great, if not greater. His mental anguish, especially the hiding of his Father's face, was so intense as to literally break his heart; but it cannot have been the same, either in character; extent, or continuance, as the everlasting pangs of conscious guilt. All that was practicable, in him as a substitute forman, was to undergo an ordeal as similar in kind and degree as his pure human nature would admit. In this sense he drank the bitter cup of atonement to its very dregs, but it was not the identical draught intended for mankind. Finally, such an absolute vicariousness would have been useless, and that in two most vital respects it would so fully have exhausted the penalty for all possible or foreseen human transgression as to render the personal punishment of any offender thereafter impossible, because unjust; and it would have been no gain or saving of suffering on the whole, but a mere shifting of a specific load from the shoulders of one being to those of another. No larger average of happiness could have resulted; nor any greater glory redounded to God. Such an atonement would have defeated instead of furthering the main design of its merciful Projector. It would have been fatal to all the advantages seen above to be secured by Christ's mediation. (See Vicarious Suffering).