Question: Why is the redemption or reconciliation of humanity with God presented to us in the form of a covenant, indeed a covenant of grace?
Answer: 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 want peace. God acts the same way toward us: in order that we might have rest and peace in our consciences, God was willing, out 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 (Isaiah 54:8-10)
7 “For a brief moment I forsook you,
But with great compassion I will gather you.
8 “In an outburst of anger
I hid My face from you for a moment,
But with everlasting loving-kindness I will have compassion on you,”
Says the Lord your Redeemer.
9 “For this is like the days of Noah to Me,
When I swore that the waters of Noah
Would not flood the earth again;
So I have sworn that I will not be angry with you
Nor will I rebuke you.
10 “For the mountains may be removed and the hills may shake,
But My loving-kindness will not be removed from you,
And My covenant of peace will not be shaken,”
Says the Lord who has compassion on you.
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.
Question: But how did Jesus Christ make the covenant between the Father and us? That is, how did He reconcile us to the Father so that our sins are eternally forgotten and the Holy Spirit and eternal life are bestowed upon us?
Answer: By His sacrifice on the cross He completely reconciled us to the Father with an eternal covenant. The Son Himself cried out on the cross that the covenant was completely ratified (“It is finished!” John 19:30), and the Holy Spirit says in Hebrews 10:14, “By one offering He has perfected forever those who are being sanctified.”
Caspar Olevianus, A Firm Foundation, p. 5