2 of the reasons that I don't agree with Calvinism are: that it seems to me to imply determinism and I cannot see how a deterministic God is not the author of sin; and I believe that saved men can subsequently reject the faith.
It is important to note that false believers, wolves in sheep's clothing, and antichrists can participate in the established church. There are people who would seem to be part of a church but who do not follow Christ and are not saved. Moreover there are men who would deceive others, draw them away from the faith, or use religion as a cover for personal gain. Some may deny Jesus is God. When Paul says to the Ephesians he fears some will arise to attack the flock (Act 20:29-30) and John speaks of those who left because they did not belong (1Jo 2:19) they are not generally speaking of apostates but of weeds. Men who have never entered the kingdom of God but who have infiltrated the church. These people may not be obvious to us, and we must be wary of removing them (Mat 13:29).
There are also apostates, those who started the journey but who no longer follow Christ. They are not those who struggle, or who are weak in the faith. They are men who had chosen the way of life, responded to the wooing of God, and turned their hand to the plow—then looked back. Those who have become entrapped in the false promises of the world once again. Having rejected God's Way they go their own way.