Amen, means "so be it," or more than "I believe." It is an expression of faith, not merely belief. from Latin, from Greek, from Hebrew 'āmēn; its Semitic roots are: A men, to be firm.
Amen is frequently used at the end of a prayer or hymn. The word is found in the Hebrew Bible and New Testament as a declaration of affirmation or approval (verily; so be it; truly; let it be; etc.) Colloquially is used to express strong agreement.
Muslims use the word Amin.
St. Matthew attributes it to Our Lord twenty-eight times, and St. John in its doubled form twenty-six times. 
May the grace of our Lord Jesus Christ, the love of God the Father, and the communion of the Holy Spirit be with you all. Amen. (II Corinthians 13:14)
- Amen Catholic Encyclopedia.