In Hebrew, the word “Amen” shares the same root as the word “believe.” This root also expresses trustworthiness and faithfulness.
When you read the gospels, you’ll see that Jesus sometimes used the word “Amen” twice in a row to emphasize the trustworthiness of his teaching. He wanted his listeners to pay special attention.
So when we say “Amen” at the end of a prayer, we reinforce our faith in what we just said. We also express our confidence that God will hear our prayers.