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38John said to him, “Teacher, we saw someone casting out demons in your name, and we tried to stop him, because he was not following us.”
39But Jesus said, “Do not stop him, for no one who does a mighty work in my name will be able soon afterward to speak evil of me.
40For the one who is not against us is for us.
41For truly, I say to you, whoever gives you a cup of water to drink because you belong to Christ will by no means lose his reward." John 9:38-41
People shouldn't despise other Christian groups which is plainly stated in the above verse. It's also worth noting that the letters of the New Testament were directed toward different groups. You will find things in some books of the Bible that were not written in other books. The recipients of these letters were not necessarily aware of the letters other church groups were getting because since churches are made of individuals who face unique challenges, it behooved the writers of these letters to tailor them to their audience.
Plainly, this is why the Bible as we know it is a collection of writings with profoundly different scriptural interpretations depending on how the reader understands them relative to their situation in life. There are non-negotiable core tenants in Christianity, but there is also a lot of freedom.
How we're told to interact with those who we are in disagreement with is with the humility to know we cannot have all of the answers unless we were blessed with all knowledge and wisdom beforehand, grace to understand that sanctification is process and accumulation of wisdom is incremental through experience, forgiveness to those with condescending pride and judgement, and above all else Love which unites us and motivates us to uplift and encourage each other as member of the body of Christ.