I have several family members that I love who affirm a gospel called Roman Catholicism. This is vexing to my soul, and it has caused me to lose many sleepless nights, because this trifling and gross tradition has not and will not ever lead one soul into heaven.
Roman Catholics do not believe that God is just and the justifier of men—not because of works—but because of Christ’s iustitia aliena (alien righteousness). They believe in faith plus works: “…so that all men may attain salvation through faith, Baptism and the observance of the Commandments” (Catholic Catechism 2068; emphasis added). This is heresy because men are not acquitted because of their works or innocence, but by the gratuitous acquittal from God, and the attestation of righteousness that Christ imputes to those that He reconciled to the Father.
The Bible teaches a different message than Roman Catholicism: The source of justification is God’s free grace, and the instrument of justification is through faith alone which is a gift of God, not of works: “For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast” (Eph. 2:8-9; emphasis added).
Roman Catholics despise Eph. 2:8-9 because they do not rest on the indefectibility of grace that God reckons to His elect because of Christ’s meritorious work. They reject it because Christ’s perfect righteousness is not sufficient for them. This is why Roman Catholicism is heresy: They boast in their fictitious traditions that are powerless to save! And with feigned lips, they curse men who do affirm justification by faith alone:
"If anyone says that the sinner is justified by faith alone, meaning that nothing else is required to cooperate in order to obtain the grace of justification, and that it is not in any way necessary that he be prepared and disposed by the action of his own will, let him be anathema" (Council of Trent, Canon 9).
There are several other reasons why Roman Catholicism is a wicked denomination. They do not believe in the sufficiency of Scripture alone that constitutes their salvation (CC 97), and they call the Pope the "head of the church" (CC 883)—which is a role that is exclusively used for Christ alone (Col. 1:18). And they egregiously think that the Pope is infallible (CC 891)—despite the fact that Scripture teaches that “none are righteous” (Rom. 3:10) and “all have sinned” (Rom. 3:23)—except Christ who is sinless and perfect (cf. 1 Pet. 2:22).
In closing, Roman Catholicism is a gospel-less tradition that cannot and will not ever save. That is because Roman Catholicism is a false gospel.
What is the true Gospel and what does it principally teach? God is the author of justification, and the source is His free grace. God the Son is the agent. And the only grounds for justification is His perfect righteousness (meritum condignum) —His alien preceptive and penal obedience—which God the Father reckons unto the account of sinners (i.e., His elect). The instrument by which the righteousness of Christ is applied to the sinner is through faith alone which unites the elect of God to their vicarious substitute and Savior.