A.T. Robertson 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)

In him is the yea (εν αυτω το Να). Supply γεγονεν from the preceding sentence, "In him was the Yea come true." This applies to all God's promises.

The Amen (το Αμην). In public worship (1 Corinthians 14:16).