This is a BIG question! First, we must understand some basic terminology.
The term Christian refers to both Roman Catholics and Protestants (which is what you refer to as "Christian"). Actually, the term "catholic" means "universal" so in this way, we are all catholic Christians. We, however, are not Roman Catholic Christians. We are Protestant Christians.
Here is a link that will help clarify the differences.
Additionally, Justin Taylor provides another good summary in his blog, Between Two Worlds.
Lastly, Matt Slick of CARM.org has a ton of articles and research that will help you understand the Roman Catholic Church and its teachings HERE. This will take some time to read, but if you're serious about this question, read it through carefully.
[Answered by Pastor HM]
