Why do some claim that Catholics can't be true Christians, or that Catholicism is a false religion?

It would be difficult not to respect most of your comments here. But I feel I must clear up one very important point. It is true that one does not have to be an actual member of the Catholic Church in order to be saved. However, in order for the Catholic Church to be the one and only true faith, we are required to believe that all who are saved are saved only through the Catholic Church by way of a very special spiritual union with the Catholic Church by following a sincere, honest and INFORMED conscience (implicit consent). The reason for this is that the Truth and God never change. There is only one reality and the Catholic Church is the only one in full possession of the truth; however, it does recognize much truth in all other religions. You give me the impression that you are one who truly and honestly follows his conscience, and only God can judge the true motives of those who are not actual members since belief in the Catholic Church is an unearned gift anyway. The best to you.