The covenant which was confirmed before of God - By God, in his promise to Abraham. It was confirmed before the giving of the Law. The confirmation was the solemn promise which God made to him.
In Christ - With respect to the Messiah; a covenant relating to him, and which promised that he should descend from Abraham. The word “in,” in the phrase “in Christ,” does not quite express the meaning of the Greek εἰς Χριστὸν eis Christon. That means rather “unto Christ;” or unto the Messiah; that is, the covenant had respect to him. This is a common signification of the preposition εἰς eis “The law.” The Law given by God to Moses on Mount Sinai.
Which was four hundred and thirty years after - In regard to the difficulties which have been felt respecting the chronology referred to here; see the note at Acts 7:6. The exact time here referred to was probably when Abraham was called, and when the promise was first made to him. Assuming that as the time referred to, it is not difficult to make out the period of four hundred and thirty years. That promise was made when Abraham was seventy-five years old; Genesis 12:3-4. From that time to the birth of Isaac, when Abraham was a hundred years old, was twenty-five years; Genesis 21:5. Isaac was sixty when Jacob was born; Genesis 25:26. Jacob went into Egypt when he was one hundred and thirty years old; Genesis 47:9. And the Israelites sojourned there, according to the Septuagint Exodus 12:40, two hundred and fifteen years, which completes the number: see Doddridge, Whitby, and Bloomfield. This was doubtless the common computation in the time of Paul; and as his argument did not depend at all on the exactness of the reckoning, he took the estimate which was in common use, without pausing or embarrassing himself by an inquiry whether it was strictly accurate or not.
His argument was the same, whether the Law was given four hundred and thirty years after the promise, or only two hundred years. The argument is, that a law given after the solemn promise which had been made and confirmed, could not make that promise void. It would still be binding according to the original intention; and the Law must have been given for some purpose entirely different from that of the promise. No one can doubt the soundness of this argument. The promise to Abraham was of the nature of a compact. But no law given by one of the parties to a treaty or compact can disannul it, Two nations make a treaty of peace, involving solemn promises, pledges, and obligations. No law made afterward by one of the nations can disannul or change that treaty. Two men make a contract with solemn pledges and promises. No act of one of the parties can change that, or alter the conditions. So it was with the covenant between God and Abraham. God made to him solemn promists which could not be affected by a future giving of a law. God would feel himself to be under the most solemn obligation to fulfil all the promises which he had made to him.