They have been lead astray by the counterfeits. Counterfeit Christianity is probably the best description I can give for a cult. Counterfeit Christianity is an imitation of real Christianity. Bible terms like "Jesus Christ, Resurrection, Salvation, and Atonement" are used by the Cult, but the various cult groups have assigned entirely different meanings to these terms.
Like counterfeit money which is sometimes difficult to detect, so it is difficult to detect counterfeit Christianity, since it looks like the real thing. Experts examining counterfeit money often hold it up to a strong light and look for identifying marks. Counterfeit Christianity also has identifying marks, which can be seen when, held up to an even stronger light, the light of God's word, the bible.
We are at an advantage if we know what to expect from a cult. Cultists are very well trained to appear "Christian", and indeed believe they are the true churches, and you need the deliverance! Therefore, be bold and ask the question, "do you believe the group you represent is the only true church on the face of the earth?"
If they reply that they are, or if they are evasive, making remarks like "Well, every church has a measure of truth but...", you have made an early detection of a Cultist.
Every true Christian, if asked the same question, regardless of his denomination, would reply that the true church is comprised of believers in the Lord Jesus Christ, and HE (not some organization) is THE WAY, THE TRUTH, AND THE LIFE, (John 14:6). No legitimate denomination would claim that they alone and their members have salvation exclusively, but the cults (the counterfeits) do.