It was common practice in Judaism at the time to finish a doxology (statement of praise to God) with the word Amen.
Amen has the meaning of truly
or it is certainly so.
1 As Paul’s letter was read aloud in the early churches, the believers were expected to say an Amen
as the reader read the word. In this way, Paul encourages the hearers to confirm the truths he has communicated in his greeting.
5 à qui soit la gloire aux siècles des siècles! Amen!