4 But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, 5 to redeem those who were under the law, so that we might receive adoption as sons. 6 And because you are sons, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!” 7 So you are no longer a slave, but a son, and if a son, then an heir through God.
Calvin goes on to comment on the phrase 'made under the law': 'Christ the Son of God, who was by right exempt from all subjection, became subject to the law. Why? In our name, that He might obtain freedom for us...a free man redeemed a slave by constituting himself a surety; by putting the chains on himself he takes them off the other'. (But Calvin adds later a significant comment: 'We are not...so exempted from the law...that we no longer owe any obedience to the teaching of the law, and may do what we please. For it is the perpetual rule of a good and holy life'.) C.S. Lewis has a fine and beautiful sentence in his book on 'Miracles' that may be profitably pondered in this connection: '...Humanity must embrace death freely, submit to it with total humility, drink it to the dregs, and so convert it into that mystical death which is the secret of life. But only a Man who did not need to have been a Man at all unless He had chosen, only one who served in our sad regiment as a volunteer, yet also only one who was perfectly a Man, could perform his perfect dying; and thus (which way you put it is unimportant) either defeat Death or redeem it. He tasted death on behalf of all other. He is the representative 'Die-er' of the universe: and for that very reason the Resurrection and the Life. Or, conversely, because He truly lives, He truly dies, for that is the very pattern of reality. Because the higher can descend into the lower, He who from all eternity has been incessantly plunging Himself in the blessed death of self-surrender to the Father can also most fully descend into the horrible and (for us) involuntary death of the body. Because Vicariousness is the very idiom of the reality He has created, His death can become ours.