Why do we say Amen?
Amen is a way to join a prayer, not simply mark its ending. The Prayer Book Catechism and Saint Paul help explain this small response.

Someone else has led the prayer. You answer Amen. In that brief response, you join what has been offered to God.
The 1662 Catechism explains Amen with the words So be it.
It is an expression of assent and hope, rather than simply a signal that the speaking has finished.
Saint Paul makes a practical point about this in 1 Corinthians 14:16–17. If a thanksgiving cannot be understood by another person, how can that person say Amen? The concern is shared worship. Others need to understand the prayer in order to join it.
That gives us a reason to listen, even to familiar words. What are we thanking God for? What are we asking? Whose need is being brought before God?
Amen does not guarantee that a request will be granted in the way we imagine. Nor must we manufacture a strong feeling before saying it. We may offer the response quietly, trusting God with a prayer we could not have composed ourselves.
At the next service, choose one prayer and listen through to its ending. Let your Amen answer that prayer. A small word can be a deliberate act of participation.


