Amen

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Old English, from ecclesiastical Latin, from Greek amēn, from Hebrew 'āmēn ‘truth, certainty’, used adverbially as expression of agreement, and adopted in the Septuagint as a solemn expression of belief or affirmation.

wiktionary

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From Middle English amen, from Old English, from Ecclesiastical Latin āmēn, from Ancient Greek ἀμήν(amḗn), from Biblical Hebrew אָמֵן‎ (ʾāmēn, “certainly, truly”) (cognate with Arabic آمِينَ‎ (ʾāmīna), Classical Syriac ܐܡܝܢ‎ (ʾāmên)). In Old English, it was used only at the end of the Gospels. Elsewhere, it was translated as sōþlīċe!(“truly”, “indeed!”), swā hit is(“so it is”), and sīe!(“[so] be it!”).

etymonline

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amen (interj.)

Old English, from Late Latin amen, from Ecclesiastical Greek amen, from Hebrew amen "truth," used adverbially as an expression of agreement (as in Deuteronomy xxvii.26, I Kings i.36), from Semitic root a-m-n "to be trustworthy, confirm, support."

Compare similar use of Modern English certainly, absolutely. Used in Old English only at the end of Gospels, otherwise translated as Soðlic! or Swa hit ys, or Sy! As an expression of concurrence after prayers, it is recorded from early 13c.