In a sermon on the acceptable sacrifice of Christ (Sermons and Discourses, 1723-1729), Edwards emphasizes that union of Christ with His people is the foundation for Christ’s substitution for His people: “Christ, that gave himself in sacrifice, is so united to them he died for, that it may well be accepted as their sacrifice. It may well be accepted of God as if we had offered that price, and justice in Christ’s suffering may well be satisfied for our sins; it may be looked upon as though we had suffered, because Christ is as we. He laid down his life out of love to us; he has been pleased to unite himself to us in his heart, to love us so as to put himself in our stead in the most extreme case.”
The location of the union is in the heart, because personal unions are necessarily unions of love, which can be so strong that lover and beloved become one subject. As Edwards says, “The husband being united to the wife, the debts of the wife may be looked upon as his debts, and his goods as her goods.” Animal sacrifices were inadequate for precisely this reason: “there was no propriety in looking upon the sufferings of the beasts slain in sacrifice, as though they were the sufferings of him that they were offered for, for there was no union of this nature between them.”
Behind union is election, the “in Christ” of Paul’s teaching on election: Christ “is appointed hereto of the Father. God chose him for our mediator; he is God’s elect, and his anointed and sealed. And therefore God looks upon him as our head, and the head being united to the members, the sufferings of the head may be looked upon as the sufferings of the members.