Why is it not a good idea to criticise other Christians ?

If you are a Christian, it is not a good idea to criticize anyone, be they also Christian, or not.

If you are a Christian, you shold follow the teachings of Jesus. Jesus said we should love one another - not go around finding fault with one another.

It depends on what you mean by criticize. We can cot Criticize their motives or hearts but their teachings we can. Paul commended the Bereans in Acts for holding his teachings in light of scripture to make sure what he taught was true. we are also told to test all teachings (hold in light of scripture) and to hold on to what is good and abstain from what is evil. If we come across a Christian who maybe in the wrong we should go to him/her and respectfully relay your reasoning with this person (with scripture to back you up) If done with gentleness and respect you wont come across self-righteous. They may however become offended. The letters to the Corinthian Church by paul was in context kind of the same thing. The Church started to adopt paganistic practices but paul wrote in a way not to say what you are doing is wrong but gave them truthful instruction and left it for them to correct themselves.

Matthew 7:4-5

"How dare you say to your brother 'Please let me take that speck out of your eye" when you have a log in your own eye? You hypocrite! First take the log out of your eye, and then you will be able to see clearly to take the speck out of your brother's eye."

It's basically because it's hypocritical. No one's perfect.

I generally just criticize their leaders when their abuses warrant it.

I'm a Deist

Evil is as evil does.

Hmmm....they will get totally ticked?