The doctrine of separation insists that Christians must separate from professing Christians who deny essential doctrines. This is the fundamentalist position. Some have a problem with the notion of ‘essential doctrines’ because it suggests that other doctrines are ‘non-essential’. That really isn’t the case, as Dave helpfully explains here:
The problem is that the word essential is sometimes used as simply meaning important, and, thus, non-essential would mean unimportant. But that’s not what the word essential means in the statement above (or normally when people use it in contexts like this). If something is essential it relates to or constitutes the essence of something. As the dictionary states, “essential implies belonging to the very nature of a thing and therefore being incapable of removal without destroying the thing itself or its character.” So, to speak of the “essential doctrines of the faith” is to talk about those doctrines which cannot be removed without destroying the faith itself or its character.
This understanding of ‘essential’ is … essential!
It isn’t that any doctrines or teaching of God from the Bible is unimportant. But the thing that makes a doctrine essential is that if someone doesn’t believe it, he is not a Christian.
I thought I might offer an analogy as a further explanation.