The Bible teaches that we are justified by grace alone, through faith alone, because of Christ alone. The Catholic faith does not accept this. Catholicism adds human righteousness to the requirements, including that we must:
But when it comes to salvation, the apostle Paul said of his own righteous works:
I consider them rubbish, that I may gain Christ and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith in Christ--the righteousness that comes from God and is by faith.
—Philippians 3:8-9 (NIV)
Where justification by faith alone is not clearly understood and taught, “another gospel” is being proclaimed. Galatians includes a stern warning about those who proclaim “another gospel.”:
I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another; but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed.
—Galatians 1:6-9 (NKJV)