The word amen is Hebrew, dating back to the Book of Numbers–but it’s not only used by Jews. When the ancient Greeks controlled the Land of Israel, the word entered the Greek language. From there, the little word made its way to other peoples and other prayers, and it continues to be used as a standard response in praying by Jews, Christians, and Muslims.
For a four-letter word, it certainly packs a punch. One talmudic commentator believed that amen was an acronym (in Hebrew, it has three letters) for “el melech ne’eman,” or “God, faithful ruler.” Another rabbinic saying posits that someone who responds “amen” to a blessing is greater than the one who recites the blessing in the first place. The word is related to the Hebrew word emunah, which in the Torah means faithfulness. When you say amen it’s like you’re saying, “I can believe that” or “hear, hear!”
The talmudic sage Rabbi Meir says that a child merits the World to Come from the day he or she says amen for the first time. And it’s also believed that if someone says “amen” with all his or her strength, all the gates of heaven will blow open, and nothing can be refused to them.