Nope. If you are arguing about Christian belief, then the Bible is a perfectly legitimate, and heck, probably the only source you can cite to back up a claim about what Jesus or God says is correct.
Few Christians believe in the pre-existence of human souls.
Those who don't may call themselves Christians, but in the eyes of God, they are not, as their belief refutes what is said by God. As Jesus said in James 2:20, claiming faith without living it is BS.
... but the vast majority of Christians disagree.
Contrary to recent events, the fact remains that Christian doctrine is not decided by a popular vote. As believers, we all ignore certain parts of scripture from time to time, it doesn't mean that it is any less wrong to do so simply because a majority of us do it.
Agreed. However, when you hold a decidedly minority view, a modest person would say something like, "I believe such and such to be Christian doctrine."
He would not state his personal opinion as being representative of the generality.