Catholics and Muslims just love to use that heresy stick as if it's going to strike fear into the hearts of the listener. Well, I've go news for you. True followers of Christ don't fear the cultism of either one of those false religions. True heresy according to scripture is teaching something different than what the apostles taught. Paul warned about following what men say above what is written.
1 Corinthians 4:6 And these things, brethren, I have in a figure transferred to myself and to Apollos for your sakes; that ye might learn in us not to think of men above that which is written, that no one of you be puffed up for one against another.
Catholics and Muslims constantly put what man teaches over what the apostles wrote. That's Scriptural heresy.