Complex sentence: Before that, the work was known only through copious quotes in early Christian writings, for the "Apocalypse of Peter" was popular and had a wide readership, though it was eventually not accepted into the Christian canon and thus remains among the apocrypha.
Split into simpler sentences: Before that, the work was known only through copious quotes in early Christian writings. The "Apocalypse of Peter" was eventually not accepted into the Christian canon and thus remains among the apocrypha.