The love of God is the cause of the atonement. But why did the love of God take this way of realizing its end? This is the question of the reason as distinguished from the cause. Notable theologians in the history of the church have taken the position that there was no absolute reason, that God could have saved men by other means than by the blood-shedding of His own Son, that, since God is omnipotent and sovereign, other ways of forgiving sin were available to Him. But God was pleased to adopt this method because the greatest number of advantages and blessings accrued from it. God could have redeemed men without the shedding of blood, but He freely chose not to and thereby He magnifies the glory of His grace and enhances the precise character of the salvation bestowed (e.g., Augustine, Aquinas, Thomas Goodwin, John Ball, Thomas Blake).
It might appear that this view does honor to the omnipotence, sovereignty, and grace of God and, also, that to posit more would be presumptuous on our part and beyond the warrant of Scripture. Is it not the limit of our thought to say that “without the shedding of blood” (Heb 9:22) there is actually no remission and be satisfied with that datum?
There are, however, certain things God cannot do. “He cannot deny himself” (2Ti 2:13) and it is “impossible for God to lie” (Heb 6:18). The only question is: are there exigencies arising from the character and perfections of God which make it intrinsically necessary that redemption should be accomplished by the sacrifice of the Son of God? It should be understood that it was not necessary for God to redeem men. The purpose to redeem is of the free and sovereign exercise of His love. But having purposed to redeem, was the only alternative the blood-shedding of His own Son as the way of securing that redemption? There appear to be good reasons for an affirmative answer.
A. Salvation requires not only the forgiveness of sin but also justification. And justification, adequate to the situation in which lost mankind is, demands a righteousness such as belongs to no other than the incarnate Son of God, a righteousness undefiled and undefilable, a righteousness with divine property and quality (cf. Rom 1:17; 3:21-22; 10:3; 2Co 5:21; Phi 3:9). It is the righteousness of the obedience of Christ (Rom 5:19). But only the Son of God incarnate, fulfilling to the full extent the commitments of the Father’s will, could have provided such a righteousness. A concept of salvation bereft of the justification which this righteousness imparts is an abstraction of which Scripture knows nothing.
B. Sin is the contradiction of God and He must react against it with holy wrath. Wherever sin is, the wrath of God rests upon it (cf. Rom 1:18). Otherwise God would be denying Himself, particularly His holiness, justice, and truth. But wrath must be removed if we are to enjoy the favor of God which salvation implies. And the only provision for the removal of wrath is propitiation. This is surely the import of Romans 3:25-26, that God set forth Christ a propitiation to declare His righteousness, that He might be just and the justifier of the ungodly. The Cross of Christ is the supreme demonstration of the love of God (cf. Rom 5:8; 1Jo 4:9-10). But would it be a supreme demonstration of love if the end secured by it could have been achieved without it? Would it be love to secure the end by such expenditure as the agony of Gethsemane and the abandonment of Calvary for God’s own well-beloved and onlybegotten Son if the result could have been attained by less costly means? In that event would it not have been love without wisdom? In this we cannot suppress the significance of our Lord’s prayer in Gethsemane (Mat 26:39). If it had been possible for the cup to pass from him, his prayer would surely have been answered. It is when the indispensable exigencies fulfilled by Jesus’ suffering unto death are properly assessed that we can see the marvel of God’s love in the ordeal of Calvary. So great was the Father’s love to lost men that He decreed their redemption even though the cost was naught less than the accursed tree. When Calvary is viewed in this light, then the love manifested not only takes on meaning but fills us with adoring amazement. Truly this is love. Those who think that in pursuance of God’s saving purpose the Cross was not intrinsically necessary are, in reality, not dealing with the hypothetical necessity of the atonement but with a hypothetical salvation. For, on their own admission, they are not saying that the actual salvation designed and bestowed could have been enjoyed without Christ but only salvation of lesser character and glory. But of such salvation the Scripture knows nothing, and no good purpose can be served by an imaginary hypothesis.
(Encyclopaedia of Christianity, Vol 1 by John Murray)