If a person did not believe their religion was true, they would not be a follower of it.
So, if a person is a member of the Jehovah's Witness faith, they must believe it is true.
If a Baptist thought the Baptist Church taught and practiced error, he would leave the Baptist faith.
If a Methodist thought the Methodist Church taught and practiced error, he would leave the Methodist faith.
If a Catholic thought the Catholic Church taught and practiced error, he would leave the Catholic faith.
All of these must believe their church teaches and stands for the truth or they would not be a member of it.
But, just because all of these people believe they follow the truth, that does not mean they are all correct. They can't all be correct.
They all claim to believe in the Bible, but they teach things that contradict each other.
If the Bible is true, and these religions contradict each other, then there must be error among them by contradicting the Bible. If the Bible contains these contradictions, then it can't be God's word.
The Bible does not contradict itself so if two religions contradict each other, one or both must be in error by contradicting the Bible.
Two things (churches, doctrines, etc.) can't contradict each other and both be true.
Just because JWs, Baptists, Catholics, etc. believe they are true, that alone does not make it so. We should search the scriptures daily to see if the things taught are true! (Acts 17:11)
I admire people when they will stand up in support of the things they believe, but I would hope they would have the integrity to investigate those things to see if they are true!
There are some good people who are Jehovah's Witnesses, Baptists, Methodists, Catholics and in every other denomination, but they can't all be correct.
Search the scriptures and see what the word of God says!