A phonetically mangled form of Greek 'antiphona' (responsive singing) — two choirs answering each other.
A rousing or uplifting song identified with a particular group, body, or cause; originally, a choral composition for church use.
From Old English "antefn" (a composition sung antiphonally), borrowed from Late Latin "antiphōna" (a psalm or hymn sung in alternate parts), from Greek "antíphōna" (things sounding in response), the neuter plural of "antíphōnos" (sounding in answer). This Greek compound joins "anti-" (against, in return), from PIE *h₂ent- (front, forehead, against), with "phōnḗ" (voice, sound), from PIE *bʰeh₂- (to speak, say). The original liturgical sense described responsive singing in church services — two choirs
'Anthem' and 'antiphon' are the same Greek word — 'antiphōna' (responsive singing) — that entered English twice: once through Old English (becoming 'anthem' by sound erosion) and once again directly from Greek (remaining 'antiphon'). The same word, two paths, two different English words.