The necessity of holding the resurrection arises from this--that man may obtain the last end for which he was made; for this cannot be accomplished in this life, nor in the life of the separated soul, as stated above (75, 1,2): otherwise man would have been made in vain, if he were unable to obtain the end for which he was made. And since it behooves the end to be obtained by the selfsame thing that was made for that end, lest it appear to be made without purpose, it is necessary for the selfsame man to rise again; and this is effected by the selfsame soul being united to the selfsame body. For otherwise there would be no resurrection properly speaking, if the same man were not reformed. Hence to maintain that he who rises again is not the selfsame man is heretical, since it is contrary to the truth of Scripture which proclaims the resurrection.The necessity is one that arises from the purpose for which God made us. It was not necessary for God to have made Eve, as though he were compelled to it; but having made Adam, it was not good for him to be alone and so out of a sort of necessity God made woman as well.
So we see that the only real necessity upon God is that which arises from himself. Once he begins to create, he must complete the work of creation. When he sends out his Word, it does not return to him void but completes the work he intends by it (Is. 55:11). And with respect to the resurrection (getting back again to the subject of our quotation for this post), since we cannot attain our last end apart from God's grace (as I mentioned briefly in my last post), it is "necessary" that God gives it to us. Now this is a slightly different order of things, because - though he gives us grace - we have free wills and may choose to reject him. But our freedom is also from God, and so his purposes are fulfilled in it, too.