Many Christian faiths believe they are the one and only true faith. All others are of Satan or, at the least, ignorance. This goes against the teachings of Jesus:
‘Now John answered Him, saying, “Teacher, we saw someone who does not follow us casting out demons in Your name, and we forbade him because he does not follow us.” But Jesus said, “Do not forbid him, for no one who works a miracle in My name can soon afterward speak evil of Me. For he who is not against us is on our side. For whoever gives you a cup of water to drink in My name, because you belong to Christ, assuredly, I say to you, he will by no means lose his reward.’ Mark 9:38-41 NKJ
There is a man who is teaching of Jesus and helping people but isn’t following Him and the apostles. The man wasn’t part of the church, Jesus’ group. The apostles thought he was wrong to do this and forbids him. Jesus, on the other hand, says the man is not doing wrong. Essentially, he teaches that whomever teaches of Jesus and rightly helps people (that is helps people in their suffering without seeking gain), is right in Jesus’ eyes and shall be rewarded. Notice that Jesus does make a distinction between the man who “gives you a cup of water” and the apostles, who “belong to Christ.” He makes a distinction between the “in” crowd and those who are “out” of the church (remember, church just means a gathering of people). However, that distinction is used to emphasize how the “out” crowd still will be rewarded if they are for Jesus and not against Him.
Exclusivity is counterproductive anyway. It creates a judgmental mind that is often too busy thinking what is “wrong” rather than what is done right. The goal of a church is to increase spirituality and decrease suffering. Exclusivity does neither. It creates an “us verses them” attitude that only increases mental dissonance when people fall out of the in crowd because of true and honest biblical study. It is best to not associate with churches that deny the universality of what Jesus taught. He didn’t teach tolerance, which has negative connotations, but rather He taught unity and oneness as long as people work rightly from Compassion. I don’t see many differences in Christian faiths. I see their shared focus on Compassion toward all.
Anymore, I cannot associate with anyone who has the same judgmental attitude the apostles wrongly had just because a person doesn’t carry the right label.