When discussing justification with a Roman Catholic, you may be asked to show them where it says you are saved by "faith alone".  They do this knowing full well that the Scriptures do not put those two words together except in James 2.  However, like the trinity, and the deity of Christ the teachings is very clear in the Scriptures.

Romans 3:28 "For we maintain that a man is justified by faith apart from works of the Law."

Here is a clear verse which shows we are justified by faith and not by works.  A response may immediately follow that the "law" here is the Mosaic law, and has nothing to do with the Roman Catholic institution.  However, this argument crumbles when Paul continues into Romans 4 and talks about Abraham and the law.  Abraham predates the Mosaic law, so Paul is not just referring to the Mosaic law.  Paul is stating that justification is a gift, given to us by faith alone (ie apart from works).

People who argue against it have no idea of the good news they are rejecting.