Why is the redemption or reconciliation of humanity with God presented to us in the form of a covenant, indeed a covenant of grace?
A: God compares the means of our salvation to a covenant, indeed an eternal covenant, so that we might be certain and assured that a lasting, eternal peace and friendship between God and us has been made through the sacrifice of His son. After a bitter quarrel, the disputants have peace of mind first and foremost when they commit and bind themselves to each other with a promise and sworn oath that on such-and-such a matter they wont peace. God acts in the same way toward us: in order that we might have rest and peace in our consciences, God was willing our of His great goodness and grace, to bind himself to us, His enemies, with His promise and His oath. He promised that He would have his only begotten Son become human and die for us, and that through the sacrifice of his Son He would establish a lasting reconciliation and eternal peace….He would be our God and bless us, that is, forgive our sins and impart to us the Holy Spirit and eternal life — and all this without any merit on our part. All we would have to do is accept the Son — promised and sent — by faith.
Caspar Olevianus, A Firm Foundation (1567) Q. 4.