Question

The argument over _______ divided the christian church.

Answer 1

The use of icons in worship. 
One was allowing people to have access to scripture. The catholic church didn't want the general public to have access to scripture for awhile and part of it is because they were preaching false things for awhile.
Some churches wanted to be about real Christianity and some were interested in other things.

Answer 2

The problem with Christianity isn't the religion itself, but the fact that its members are all human.
Originally, there was one Church, what we now know as the Catholic Church. However, during the 1500s, the priests of the time were corrupt (as mankind tends to be) and were charging people money in order to forgive them of their sins (as it was, nobody needed a priest to forgive them, but the people didn't know because they didn't have bibles).

And so, in 1517, Martin Luther published Ninety-Five Thesis, critiquing the Church, and soon the Church was divided between Protestant and Catholics. All the other denominations you see out there come from Protestantism.

None is better than the other. While I feel that the Protestant reformation was necessary, that does not mean that modern Catholics are necessarily bad. There are corrupt people in all churches. Meanwhile, the Christian community is supposed to be acting as one body, so any hostility you see between denominations is, by Christian standards, wrong.