Justin Bieber -- I'm NOT KIDDING About Retiring

Justin Bieber is NOT backing off his promise to "retire" after his album drops ... and last night, he proved he means business by reinforcing his retirement plans ON TWITTER!!!

Just one week after he initially announced his upcoming retirement while appearing on an L.A. radio station, Justin issued a Christmas Eve statement saying, "My beloved beliebers I'm officially retiring."

He added, "The media talks a lot about me. They make up a lot of lies and want me to fail but I'm never leaving you, being a belieber is a lifestyle."

And in a Jesus-esque final statement ... he concluded, "Be kind loving to each other, forgive each other as god forgave us through Christ Merry Christmas I'M HERE FOREVER."

Though Jesus never urinated in a mop bucket ...