in response to my comment It's not enough to "believe the Bible." Mormons, JWs, Moonies, and unitarian pentecostals all "believe the Bible." Christians believe what the Bible teaches as well.
Is it just a coincidence that cults uniformly violate one or more of the standards mentioned in the creeds? Or is it possible that we can learn from godly folks who have gone before us, in order to go further in our own lives? You might enjoy G K Chesterton's short story "The Broken Sword." The villain "read his own Bible" by himself -- and found in it such things as harem keeping! The bottom line is, that we as Christians are a body, we need one another, and we need to read God's word together!
I agree that believers need to fellowship (Eph. 3:9; Phil. 1:3-5) and that fellowship can actually be ministry (2Cor. 8:4). But fellowship can be potentially harmful and disruptive if the believers aren't like minded. We're even told to separate ourselves from those who claim to be believers, but engage in activities in their churches that lead you to believe otherwise. As you state, they may use the Bible, and even believe the Bible, but the focus is not on the Lord Jesus Christ. That doesn't mean to become hermits, but to fellowship with those you agree doctrinally with.