Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree. (Galatians 3:13)
The whole pith and marrow of the religion of Christianity lies in the doctrine of substitution, and I hesitate not to affirm my conviction that a very large proportion of Christians are not Christians at all, for they do not understand the fundamental doctrine of the Christian creed; and alas! there are preachers who do not preach, or even believe, this cardinal truth. They speak of the blood of Jesus in an indistinct kind of way, and descant upon the death of Christ in a hazy style of poetry, but they do not strike this nail on the head, and lay it down that the way of salvation is by Christ's becoming a substitute for guilty man.
This shall make me the more plain and definite. Sin is an accursed thing. God, from the necessity of his holiness, must curse it; he must punish men for committing it; but the Lords Christ, the glorious Son of the everlasting Father, became a man, and suffered in his own proper person the curse which was due to the sons of men, that so, by a vicarious offering, God having been just in punishing sin, could extend his bounteous mercy towards those who believe in the Substitute. C. H. Spurgeon, Christ Made A Curse For Us, May 30, 1869
*See also Spurgeon's sermon on 2 Corinthians 5:21, titled Substitution