Paul argues finally from the promise of the inheritance made to Abraham:
v. 15. Brethren, I speak after the manner of men. Though it be but a man's covenant, yet if it be confirmed, no man disannulleth, or addeth thereto.
v. 16. Now to Abraham and his Seed were the promises made. He saith not, And to seeds, as of many; but as of one, and to thy Seed, which is Christ.
v. 17. And this I say, that the covenant, that was confirmed before of God in Christ, the Law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
v. 18. For if the inheritance be of the Law, it is no more of promise; but God gave it to Abraham by promise.
The apostle here offers the mystery of God in a human parable, incidentally addressing the Galatians in a kind and captivating manner, to win them by his confidential tone: After the manner of man speak I. In his endeavor to show that the promise alone brings salvation, he uses a comparison taken from the ordinary practice in regard to the last will or testament of a man, by which he disposes of his goods: Though it be hut a man's, yet if it is ratified, no man sets aside a testament or adds thereto. If a man's last will and testament is duly witnessed and sealed, the disposal of his property is commonly regarded as being consummated: how much more, then, ought this to be true of the testament of God by which He made Abraham and all his children heirs of the evangelical blessing! See Heb_6:17-18. Of this testament of the Lord the apostle now says: But to Abraham were spoken the promises and to his Seed. God's testament consists of promises of grace and blessings which are not connected with any legalistic demands and conditions, such as Gen_13:15; Gen_17:8; Gen_22:18. The testament of God was, moreover, not exclusively for Abraham, was not exhausted in him, but included his Seed as well. The blessing in this Seed of Abraham is in force today, is applicable to all true children of Abraham up to the present time, for they represent all nations of the earth. For that reason Paul argues from the singular form of the noun in the Hebrew test, Gen_12:3 : He does not sap. And to seeds, as of many: but as of one, And to thy Seed, which is Christ. In all the divine promises concerning the Seed, as early as Gen_3:15, where the Messiah, through whom God wants to bless all nations, is designated, the Lord always speaks in the singular. In this one descendant of Abraham, in Jesus of Nazareth, all nations are blessed. Note that the argument of Paul, being based upon a single word in the Old Testament, is a powerful argument for the verbal inspiration of the Bible.
The statement of v. 16 had been made by way of parenthesis. The apostle now names the point which he intended to emphasize by his comparison: But this I say, A testament, ratified by God to Christ, the Law, which came into being four hundred and thirty years later, does not render void that it should invalidate the promise. The testament and will of God, the evangelical promises, were by God sealed to Abraham and therefore to Christ, who was expressly mentioned in the blessing. Some four hundred and thirty years later, Exo_12:40, counting from the journey of Jacob into Egypt to the exodus of the children of Israel, the Law was given by God from Mount Sinai. It is self-evident that this later revelation cannot annul or invalidate the promise given to Abraham. The Mosaic Law is not a codicil which sets aside the testament of the Lord, the Gospel promise given to Abraham. For, as the apostle argues: If out of the Law the inheritance, then no more out of promise: but to Abraham through promise God has freely granted it. If the spiritual inheritance, the grace and mercy of God, were actually obtained through the keeping of the Law, then the promise would no longer be in power, for obviously the two cannot be in force at the same time, that the inheritance is a free gift, and that we are still under obligation to earn it by works. But now the inheritance was a present to Abraham by the promise, by the testament of God; therefore the other assumption as to the earning of its blessings by works cannot stand. It is all free grace on the part of God, and His promise is a means of grace which does not speak of a possible good fortune which might come to Abraham, but of a transmission of the inheritance by virtue of the testamentary disposal; it is not a dead letter, but it is spirit, life, and power. Thus Paul has proved the inferiority, the subordinate character, of the Law.