The NT contains several references to Christians abandoning Christianity. They are referred to as having "turned aside after Satan" ( 1 Tim 5:15 ), as being like "the dog that is turned to his own vomit." ( 2 Peter 2:22 ), as being the "enemies of the cross of Christ" ( Php 3:18 ), and as being "not of us" ( 1 John 2:19 ).
As the early Christian congregation believed itself to be "the pillar and ground of the truth" ( 1 Tim 3:15 ), the true religion, its no surprise that they referred to those who left Christianity in the above unflattering terms, after all, they believed that they were right, and that therefore anyone who left Christianity must be wrong, and have fallen prey to Satan.
Had the internet have been available in those days, those exiting Christians may have set up a discussion board ( JWD - Jesus Witness discussion? ) Had you have been one of those disillusioned Christians ( as I suspect I would have been ) what reason would you have posted on the site, as to why you lost faith in Christianity, and felt that you had no choice but to walk away from it?