I don't think you can call the Catholic Church a sect, dear.
They were only one of many Christian groups that evolved after the death of Jesus. They won the battle and were the only christian game in town between the Council of Nicea and the Reformation (unless you count the schism of eastern and western orthodoxy in 1054), but they were one of many groups claiming to be spinning Truth. See also, Ebionites, Marcionites, the Gnostics, and the Arian controversy.
For anyone interested in early Christianity, Lost Christianities, The Battles for Scripture and the Faiths We Never Knew
by BD Ehrman with the companion book Lost Scriptures
is an interesting read. I also recommend his latest book Misquoting Jesus
for a discussion of how the gospels were modified over time.
There was John the Baptist, son of Elizabeth....and then there was John the Apostle one of the 12 disciples...confusing, yes, lots of Mary's and Johns around then....but the latter John is the one that is normally given credit for writing "John" and as being..."the one that Jesus loved".
On the origins of the Gospel of John... From The Complete Gospels
Robert J Miller, editor -- "Where and when was this gospel written? It is addressed: to an audience of whom part are (or until recently were) bilingual, thinking in both Greek and 'Hebrew'; to a city having a substantial and effective Jewish presence, over against which those 'who were born from God' and 'have believed in his name' understand themselves as a beleaguered but divinely vindicated minority; and to a perspective not far removed from that of Galilee and Judea. A small city in Syria is perhaps the best informed guess. And the date would be within the decade or two following the centralized Jewish decision to expel believers in Jesus from the synagogue, that is, during the last fifteen years or so of the first century -- roughly contemporary with, but evidently independent of, the writing of Matthew and Luke-Acts. The author, like the three other 'evangelist,' is anonymous and only a century later was identified with John, the son of Zebedee (and he with "the disciple Jesus loved most").