God did this so that the righteous demands of the Law might be fully satisfied in us who live according to the Spirit, and not according to human nature. Rom. 8:4, T.E.V.
A young man travels a great distance from home and falls in love with a delightful young woman. In order to win her hand in marriage, he works years for an unpleasant employer--who happens also to be the girl's father. In the end, the father pulls a classic dirty trick and switches daughters on the young man.
Not one of us today would think that what happened to Jacob was fair. You just cannot substitute one relationship for another. Laban may have argued that one wife was as legally proper as another, but that argument would have been wasted on Jacob. He knew the effects in his own heart of being in love with Rachel.
I am just as unhappy with any picture of God that suggests He is satisfied with a substitute relationship. Our Father created each one of us for a unique and personal relationship with Himself. The central purpose in our creation can be fulfilled only as we enter into that relationship. Would the Father be satisfied by substituting His relationship with Jesus in place of our own, claiming that this fulfills a "legal" requirement? Wouldn't that ignore the personhood of God and of His friends?
To be in a confident, trusting union with God, as the branch is to the vine, is the only right and proper condition of man. This is righteousness; there can be no substitute for it, for it references to the hearts of God and man, not to a legal code.
Many people think of righteousness by faith strictly in legal terms--seeing it as God legally substituting Jesus' righteous record in place of our own, and thereby being content that the sin problem has been solved! But we deal better with the real problem by seeing faith as that deep, trusting, submitted friendship with God, which alone is our rightful state.
God deals with us in order to end the separation rather than to deal with legal "as ifs." His concern is with restoring relationships, not with balancing books. No wonder Jesus was eager to present Him to us as a father rather than as a judge. The "just requirement of the law" (Rom. 8:4, R.S.V.) is union with the Father. Thus Jesus is the way to the Father, not a substitute for it!