What is the X in X-mas?
Old English, from Late Latin amen, from Ecclesiastical Greek amen, from Hebrew amen "truth," used adverbially as an expression of agreement (e.g. Deut. xxvii:26, I Kings i:36; cf. Modern English verily, surely, absolutely in the same sense), from Semitic root a-m-n "to be trustworthy, confirm, support." 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.
This Hebrew word means firm, and hence also faithful (Rev. 3:14). In Isa. 65:16, the Authorized Version has "the God of truth," which in Hebrew is "the God of Amen." It is frequently used by our Saviour to give emphasis to his words, where it is translated "verily." Sometimes, only, however, in John's Gospel, it is repeated, "Verily, verily." It is used as an epithet of the Lord Jesus Christ (Rev. 3:14). It is found singly and sometimes doubly at the end of prayers (Ps. 41:13; 72:19; 89:52), to confirm the words and invoke the fulfilment of them. It is used in token of being bound by an oath (Num. 5:22; Deut. 27:15-26; Neh. 5:13; 8:6; 1 Chr. 16:36). In the primitive churches it was common for the general audience to say "Amen" at the close of the prayer (1 Cor. 14:16). The promises of God are Amen; i.e., they are all true and sure (2 Cor. 1:20).