In 1:19 and 20 Paul speaks concerning the unchanging Christ with God’s promises in Him: “For the Son of God, Christ Jesus, who was preached among you through us, through me and Sylvanus and Timothy, did not become yes and no, but in Him is yes. For whatever promises of God there are, in Him is the Yes; wherefore also through Him is the Amen to God, for glory through us.” “For” gives the reason for what is mentioned in the preceding verse. God is faithful, never changing, especially in His promises concerning Christ, so that the word the apostles preached concerning Christ is likewise never changing, because the very Christ whom God promised in His faithful word and whom they preached in their gospel did not become yes and no, but in Him is yes. Since the Christ whom they preached according to God’s promises did not become yes and no, the word they preached concerning Him was not yes and no. Not only their preaching but also their living was according to what Christ is. They preached Christ and lived Him. They were not men of yes and no, but men who were the same as Christ.
Again in verse 20 “For” gives the reason for what is mentioned in the preceding verse. Christ, whom the faithful God promised and whom the sincere apostles preached, did not become yes and no; that is, He did not change. The reason Christ did not change is that in Him is the Yes of all the promises of God, and through Him is the apostles’ and the believers’ Amen to God for His glory.
In verse 20 Paul says that Christ is the Yes. Christ is the Yes, the incarnate answer, the fulfillment of all the promises of God to us.
The Amen in verse 20 is the Amen given by us through Christ to God (see 1 Cor. 14:16). Christ is the Yes, and we say Amen to this Yes before God. This is for the glory of God. When we say Amen before God to the fact that Christ is the Yes, the fulfillment, of all the promises of God, this is a glory to God through us.
The “us” in verse 20 refers not only to the apostles, who preached Christ according to God’s promises, but also to the believers, who received Christ according to the apostles’ preaching. Through both there is glory for God when they say Amen to Christ as the great Yes of all God’s promises.