Albert Barnes Commentary 2 Corinthians 1:20

"For how many soever be the promises of God, in him is the yea: wherefore also through him is the Amen, unto the glory of God through us." — 2 Corinthians 1:20 (ASV)

For all the promises of God in him. This means all the promises God has made through Him. This is another reason why Paul felt himself bound to maintain a character of the strictest veracity. The reason was that God always demonstrated this quality; and because none of His promises failed, Paul felt himself sacredly bound to imitate Him and to adhere to all his own promises. The promises of God, which are made through Christ, relate to the pardon of sin for the penitent, the sanctification of His people, support in temptation and trial, guidance in perplexity, peace in death, and eternal glory beyond the grave. All of these are made through a Redeemer, and none of these will fail.

Are yea. This means all will be certainly fulfilled. There will be no vacillation on God's part; no fickleness; no abandoning of His gracious intention.

And in him Amen. In Revelation 3:14, the Lord Jesus is called the Amen. The word means true, faithful, certain; and this expression here means that all the promises made to people through a Redeemer will be certainly fulfilled. They are promises that are confirmed and established, and that will by no means fail.

Unto the glory of God by us. This means either by us, ministers and apostles, or by us who are Christians. The latter, I think, is the meaning. Paul means to say that the fulfillment of all the promises God has made to His people will result in His glory and praise as a God of condescension and veracity. The fact that He has made such promises is an act that tends to His own glory—since it was by His mere grace that they were made. Furthermore, the fulfillment of these promises in and through the church will also tend to produce elevated views of His fidelity and goodness.