so, what is the difference between, catholics, presbytarian, lutheran, anglican, born again christians, baptists etc. or perhaps i could ask,, what are the similarities.
Well i can give you a wholly biased Catholic viewpoint:
Assuming the 'Church' was founded on Pentecost, around 33AD (the descent of the Holy Spirit upon the disciples), the 'orthodox' (Gk: ortho = 'right' and doxa = 'thinking') doctrine was that taught by the Apostles and their successors — they're the ones who got it from the horse's mouth, as it were.
The Church soon settled into a common profession of faith — a life founded in and focussed on the liturgy and the sacraments. From this teaching emerged the Gospels and the Letters that comprise the New Testament.
As the Roman Empire began to fragment, local and national interests began to impact upon theology, so that what theology became the rule became something of a national interest.
The first really big dispute, the Arian crisis (Arius taught that Christ is created, 'there was a time when he was not', whereas the teaching of the Church until then was that Christ was eternally in the Father). The dispute very quickly became a political one, and even Arius noted that no-one was really interested in what he believed, what was more important was poilitiocal influence in the Church ... something the Church resisted.
Later schisms, such as the Copts, the Nestorians etc., emerged along geo-political boundaries as much as theological difference — I am in discussion with a Copt on another site and we both hold somewhat simplified and wholly erroneous views about the other's theology ... in fact now the Copts are united with Rome, although they still have local autonomy under their own Pope ... Nestorians are another kettle.
Then The Reformation which was wholly motivated by political concerns — Martin Luther objected to moneys being raised in the German states that went to Rome. (He had no problems with indulgences when they were financing his local bishop)
Luther also had deep psychological issues, one being he could not believe that Christ could forgive us our sins ... so he developed a theology in which some were pre-destined to be saved, others pre-destined for damnation.
Very quickly, Luther got left behind as others followed his example and declared their own versions of Christianity ... Calvin 'justified' the emerging Swiss bourgeoise middle classes, for example...
And so it goes on ... in my view the two things that mark the differences between denominations are:
1 - Geopolitical determinations (The Greek Orthodox Church is now the religion of state, to such a degree that theological decisions have to be viewed according to how they might impact on Greek nationalism — and of course the Christian denominations of the US variety assume that the 'saved' will be drawn from their own race and class.)
2 - Rationalism — each denomination tends to 'reduce' the element of the Mysteries central to traditional and orthodox Christianity to something more rational and acceptable to its own powers of reason and logic ... in short they explain the mystery away ... what you're left with bears a greater resemblance to humanism than anything else — Christ is reduced to a sentimental 'nice guy' who paid a high price for being a good bloke.
So modern Christianity regards traditional Christianity as subject to myth, superstition and ritual ... whereas traditional Christianity looks at modern Christianity and sees and empty shell ...
Thomas