I'm not surprised Reg is being so doctrinaire but I am surprised that he seems to refuse to acknowledge the existence of large numbers of people who would call themselves Christians, who do not believe in the divinity of Christ or in a 'God'.
Take just this guy for example. I don't have to offer any more evidence than this because you only need one exception to disprove a proposition. And if the proposition is that 'All Christians believe in God', then this is clearly false. Now if Reg is actually saying 'All true Christians believe in God' then he's just fallen in a big logical hole - this is
the famous 'No true Scotsman' fallacy. Of course, Reg may now go on to assert this is a matter of fact not opinion, but if there's one thing clear about religious doctrinal arguments that we know from hundreds of years of conflict, heresy, schism, inquisition etc., it is that all of these facts are subject to interpretation, revelation and transformation in different contexts. Asserting that there is something called 'the fundamental teachings of Christ' which is an interesting opinion about what those facts are in itself...