Why is it difficult? Not at all. The scripture is filled with Catholic doctrine everywhere you look. There are things not in the scripture, and for their propaganda purposes the anti-Catholics want to talk about that, but if you stick to the things that the Scripture contains, it is all Catholic.
Well no, scriptures certainly are not ALL catholic as these threads do attest to. But of course it is quite understandable a catholic such as yourself might see it as so.