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.
late 14c., "at the coming of the Lord," a Bible word, from Greek maranatha, a Greek form of an untranslated Aramaic (Semitic) word in I Corinthians xvi.22, where it follows Greek anathema (with which it has no grammatical connection), and therefore has been taken as part of a phrase which is used as a curse (see anathema). The Aramaic word has been explained as "Our Lord, come thou" or "Our Lord hath come," apparently a solemn formula of confirmation, like amen; but possibly it is a false transliteration of Hebrew mohoram atta "you are put under the ban," which would make sense in the context [Klein].