Many Protestants treat Roman Catholicism as just another Christian denomination. “Sure,” they say, “we might disagree with them about a large number of secondary issues, but when it comes to the gospel, we are in basic agreement.” But these people are uninformed about the true nature of Catholicism and how their tradition distorts the simplicity of the gospel. It is on the issue of the gospel, rather than moral issues, where there is the most disagreement.
1. The Council of Trent anathematizes all Protestants who don’t agree with Roman Catholic doctrine. To give one example:
“But if any one receive not, as sacred and canonical, the said books [includes deuterocanonicals] entire with all their parts, as they have been used to be read in the Catholic Church, and as they are contained in the old Latin vulgate edition; and knowingly and deliberately contemn the traditions aforesaid; let him be anathema” (4th Session).
This was “corrected” at Vatican II and now Protestants are seen as “separated brethren” rather than as anathematized heretics.
2. Indulgences (even plenary ones) are still practiced in Roman Catholicism:
“An indulgence is a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints. ‘An indulgence is partial or plenary according as it removes either part or all of the temporal punishment due to sin.’ The faithful can gain indulgences for themselves or apply them to the dead” (Catechism 1417).
Here is one modern example.
3. They believe Muslims worship the same God we do and that people can be saved apart from faith in Christ:
“The plan of salvation also includes those who acknowledge the Creator, in the first place amongst whom are the Muslims; these profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind’s judge on the last day” (Catechism 841).
“Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience – those too may achieve eternal salvation” (Catechism 847).
Compare that with 1 John 2:23: “No one who denies the Son has the Father. Whoever confesses the Son has the Father also.” Muslims deny the Son because they reject the crucifixion and resurrection of Christ, not to mention his deity.
Needless to say, the author of Hebrews would disagree: “And without faith it is impossible to please him, for whoever would draw near to God must believe that he exists and that he rewards those who seek him” (Heb 11:6).
5. Catholics worship the consecrated host during the Mass:
“Wherefore, there is no room left for doubt, that all the faithful of Christ may, according to the custom ever received in the Catholic Church, render in veneration the worship of latria, which is due to the true God, to this most holy sacrament” (Trent, Session 13, Chapter 5).
“If any one saith, that, in the holy sacrament of the Eucharist, Christ, the only-begotten Son of God, is not to be adored with the worship, even external of latria; and is, consequently, neither to be venerated with a special festive solemnity, nor to be solemnly borne about in processions, according to the laudable and universal rite and custom of holy church; or, is not to be proposed publicly to the people to be adored, and that the adorers thereof are idolaters; let him be anathema” (Trent, Session 13, Chapter 5).
“Worship of the Eucharist. In the liturgy of the Mass we express our faith in the real presence of Christ under the species of bread and wine by, among other ways, genuflecting or bowing deeply as a sign of adoration of the Lord. ‘The Catholic Church has always offered and still offers to the sacrament of the Eucharist the cult of adoration, not only during Mass, but also outside of it, reserving the consecrated hosts with the utmost care, exposing them to the solemn veneration of the faithful, and carrying them in procession'” (Catechism 1378).
“Because Christ himself is present in the sacrament of the altar, he is to be honored with the worship of adoration. ‘To visit the Blessed Sacrament is . . . a proof of gratitude, an expression of love, and a duty of adoration toward Christ our Lord’ (Paul VI, MF 66)” (Catechism 1418).
“The Catholic Church has always displayed and still displays this latria that ought to be paid to the Sacrament of the Eucharist, both during Mass and outside of it, by taking the greatest possible care of consecrated Hosts, by exposing them to the solemn veneration of the faithful, and by carrying them about in processions to the joy of great numbers of the people” (Mysterium Fidei).
6. They believe we can merit eternal life:
“Since the initiative belongs to God in the order of grace, no one can merit the initial grace of forgiveness and justification, at the beginning of conversion. Moved by the Holy Spirit and by charity, we can then merit for ourselves and for others the graces needed for our sanctification, for the increase of grace and charity, and for the attainment of eternal life” (Catechism 2010).
7. They believe the Holy Spirit is present in the heart of every person:
“The Holy Father explained [that] the Holy Spirit is mysteriously present in the heart of every person. Through the practice of what is good in their own religious traditions, and following the dictates of their consciences, members of other religions positively respond to God’s invitation and receive salvation in Jesus Christ, even though they may not recognize Him as their Savior” (VIS, 9/9/98).
8. They believe Mary’s intercessory work is necessary for the cross to not be emptied of its power:
“O Mary, Mother of Mercy, watch over all people, that the Cross of Christ may not be emptied of its power” (Veritatis Splendor).
These are a few examples, but many more could be added. For more information, I recommend William Webster’s book The Church of Rome at the Bar of History.