I don't know any Christians who think their church has the *fullness of Truth*. The only place THE Truth can be found is in Scripture itself.
If a person believes they've found the truth in a denomination, why would they not choose to belong?
I can't see any reason why they wouldn't but again, truth is found in God's word, not a denomination.
Jesus is the only name under heaven by which we must be saved, not any church or denomination.
That is why I love my church. The pastor has been there since 1978, teaches directly from the Bible, etc.