How can a Catholic best safeguard his faith? A Catholic can best safeguard his faith by making frequent acts of faith, by praying for a strong faith, by studying his religion very earnestly, by living a good life, by good reading, by refusing to associate with the enemies of the Church, and by not reading books and papers opposed to the Church and her teaching.
I know that after my departure fierce wolves will get in among you, and will not spare the flock. And from among your own selves men will rise speaking perverse things, to draw away the disciples after them. (Acts 20:29-30)
How does a Catholic sin against faith? A Catholic sins against faith by apostasy, heresy, indifferentism, and by taking part in non-Catholic worship.
Why does a Catholic sin against faith by taking part in non-Catholic worship? A Catholic sins against faith by taking part in non-Catholic worship when he intends to identify himself with a religion he knows is defective.
This is why I was born, and why I have come into the world, to bear witness to the truth. Everyone who is of the truth hears my voice. (John 18:37)
Further reading on our non-Catholic brothers and sisters and working toward unity: CCC 817-