There are thousands of Christian denominations, replete with falsehood and error. This isn’t unreasonable, it’s inevitable: God isn’t easily found or understood, yet He holds each of us accountable to seek and find Him for ourselves. (Ac 17:27) We may neither delegate this responsibility, nor force our beliefs on others. In seeking God independently, we’re all at various stages of our journey, with unique levels of understanding. We all have some truth, and we all have some error.
Our goal in spiritual community, where we find all social classes and cultures, is simply stated: to edify one another, helping each other in our journey after God. (Ep 4:11-13) Being diverse in our ways, gifts, dispositions and wounds, we must defer to one another in matters of preference. (Ro 14:4-5) Yet our goal implies that in matters of truth we are to be seeking unity through thoughtful like-mindedness:  encouraging each other where we agree,  humbly and prayerfully challenging each other where we differ, and  seeking wisdom in each other as we’re searching and growing. (Ro 15:5)
Yet many walk among us who are not on this journey (Php 3:18-19), not seeking God, not seeking truth (2Th 2:10-12), false brothers who exploit the power of religion, enslaving themselves and others. (Ga 2:4) They company with us, enjoying the benefits of community, and appear so much like the rest of us (2Co 11:13-15) that we cannot always tell one from another with certainty; it’s not even our place … only God can do this perfectly. (Mt 13:28-30)
Even so, in pursuing the goal of spiritual community, God calls us to mark out and avoid those who either  express willful contempt for God’s laws (Mt 18:15-17, 1Co 5:12-13) or  offend and divide through false doctrine. (Ro 16:17) This must be done kindly but firmly, with wisdom, not as individuals but as a body indwelt by God. (2Co 6:16)