Good answer. Some think Gregorian chant was devised by monks around 800AD and retro-attributed to Gregory the Great who died in 604. I suspect the use of hymns by Christians is an extension of an older tradition, one that goes way back. Hymns were sung in Greek and Roman temples, but even those civs were continuing a tradition started by earlier Middle Eastern civs.
the earliest i know is that it did not became official until pope gregory made it official. he set down certain hymns for all to sing and these became the gregorian chants.