All the promises of God find their Yes in him. That is why it is through him that we utter our Amen to God for his glory. (2 Corinthians 1:20)

Prayer is the place where the past and future are linked repeatedly in our lives. I mention this here because Paul links prayer with God’s Amen in this verse in a striking way.

In 2 Corinthians 1:20, he says (with choppy Greek that comes through in choppy English), “That is why it is through him that we utter our Amen to God for his glory.” Let’s try to smooth that out.

Here’s what he is saying: “Therefore, because of Christ, we say Amen to God in our prayers to show that God gets the glory for the future grace we are counting on.”

If you’ve ever wondered why Christians say Amen at the end of our prayers and where that custom comes from, here’s the answer. Amen is a word taken straight over into Greek from Hebrew without any translation, just like it has come into English and most other languages without any translation.

In Hebrew, it was a very strong affirmation (see Numbers 5:22; Nehemiah 5:13; 8:6) — a formal, solemn, earnest “I agree,” or “I affirm what you just said,” or “This is true.” Most simply, “Amen” means a very earnest Yes in the context of addressing God.
Now notice the connection between the two halves of 2 Corinthians 1:20. The first half says, “All the promises of God find their Yes in him.” The second half says, “That is why it is through him that we utter our Amen to God for his glory.”
When you realize that “Amen” and “Yes” mean the same thing, here’s what the verse says: In Jesus Christ, God says his Yes to us through his promises; and in Christ we say our Yes to God through prayer.
– John Piper