This is a topic that seems to be gathering some momentum lately within The Salvation Army. To characterise the debate as being only between 'liberals' and 'conservatives' would be to grossly oversimplify the matter - and also to marginalise those who don't see themselves fitting under either of those labels. The basic question is 'How much room exists for differing theological opinions within our movement?'
Knowing when we've crossed the line is somewhat dependent on who gets to draw the line. Personally, I take a broad historical perspective and acknowledge that the church has always been in the process of defining what (and therefore who) is in and out. Whilst this creates its own difficulties by excluding people, it also recognises that there has never been a single correct, pure or authorative form of Christianity or for that matter of Salvationism.
I suspect that everyone has a slightly different idea of what the essentials are, those things that we cannot possibly do without. For me, Jesus has to be central. I'm particularly interested in the renewal of historical Jesus studies but I don't see them as capturing everything that is important about the Christian faith. There's also something important here about judging people by their fruits. I'm as far from perfect as the next person and my life is full of faults and unworthy behaviours. However, my purpose in following Jesus is to make this world a better place and hopefully I will one day be able to show that I've done more good than harm. Don't judge my theology (or anyone elses for that matter) by the clumsy words that I use to describe it, but perhaps instead by the way that our theology shows itself in practice.