Jamieson, Fausset and Brown on 2 Corinthians 1:20

20. Rather, How many soever be the promises of God, in Him is the "yea" ("faithfulness in His word": contrasted with the "yea and nay," 2Co 1:19, that is, inconstancy as to one's word).

and in him Amen--The oldest manuscripts read, "Wherefore through Him is the Amen"; that is, In Him is faithfulness ("yea") to His word, "wherefore through Him" is the immutable verification of it ("Amen"). As "yea" is His word, so "Amen" is His oath, which makes our assurance of the fulfilment doubly sure. Compare "two immutable things (namely, His word and His oath) in which it was impossible for God to lie" (He 6:18; Re 3:14). The whole range of Old Testament and New Testament promises are secure in their fulfilment for us in Christ.

unto the glory of God by us--Greek, "for glory unto God by us" (compare 2Co 4:15), that is, by our ministerial labors; by us His promises, and His unchangeable faithfulness to them, are proclaimed. Conybeare takes the "Amen" to be the Amen at the close of thanksgiving: but then "by us" would have to mean what it cannot mean here, "by us and you."