Roman Catholics, the Eastern Orthodox, and Protestants share many core Christian beliefs, particularly with regard to the Trinity and the Incarnation, especially as they are addressed in the ancient ecumenical councils. That being said, faithful Roman Catholics hold to several key distinctives.
One is the belief that the Roman Catholic Church is the one true church. This connects with the view that the Pope occupies the episcopal seat of Peter and is the sole vicar of Christ upon earth. This vicarious status holds several ramifications for Roman Catholic views of pastoral authority, politics, sacramental ministry, and Scripture.
Roman Catholic theologians have defended the universality of their church’s doctrine by several means. One approach is to hold to an ancient oral tradition that existed alongside the written tradition of Holy Scripture. Both the oral and written tradition coexisted together, with the oral tradition giving the definitive interpretation and application of biblical texts (such as Matthew 16:18). They believe, in and of themselves, the Holy Scriptures are not a sufficient guide and authority with regard to salvation. In the 1800s, Cardinal John Henry Newman (a high-profile convert from Anglicanism) argued for the “development of doctrine,” in which the Holy Ghost infallibly guided and guides the Roman Catholic Church toward dogmatic truth.
Roman Catholics believe in purgatory, a state in the afterlife in which a Christian’s sins are purged away, typically through suffering. This includes punishment for sins committed in one’s earthly life. It may be helpful for Protestants to understand purgatory as sanctification extended even after death, until one is truly transformed and glorified in perfect holiness. All those in Purgatory will reach heaven eventually. They do not remain there permanently, and they are never sent to the Lake of Fire.
Roman Catholics also hold to the idea of the “treasury of merit.” Roughly speaking, this is a sort of “bank” of grace, in which the merits of Jesus Christ and His holy saints are stored and can be accessed for the benefit of other Christians. It is inexhaustible due to Christ’s own infinite merit. Roman Catholics will pray to Christ or any variety of saints, beseeching them for such benefits. It is important to remember that Roman Catholics do not understand themselves to be worshipping the saints; they seek to honor them (dulia) while recognizing God alone as worthy of divine worship (latria). Protestants are typically skeptical of this distinction. One of the major controversies during the Protestant Reformation on the Pope’s claim to special access to the treasury of merit. In particular, the popes claimed that one could obtain indulgences from the Church, which could reduce the temporal punishment due for sins committed on earth. This meant shortening one’s time in Purgatory. These indulgences could be obtained for oneself or a loved one. What is more, the popes allowed for the sale and purchasing of indulgences, typically to help raise funds for their magnificent buildings and other projects. This enraged many theologians and pastors, including Martin Luther. Indulgences are still issued today, even though they are not commercialized as they were in the late medieval era thanks to reforms made in the Counter-Reformation.
With some exceptions, the Roman Catholic Church requires that her clergy be celibate. This has been a mandatory policy since the Fourth Lateran Council (1215). The Fourth Lateran Council also mandated private oral confession for sin to a priest at least once a year (as well as participating in Holy Communion annually).
That same council prescribed transubstantiation as the authoritative understanding of the Eucharist. Transubstantiation is the belief that, when a priest says the words of institution, the bread and wine in Holy Communion changes in substance to become the body and blood of Christ. The elements are no longer bread and wine; those features are simply accidents. The essence of these elements has been transformed.
This belief serves as the basis and justification for the practice of Adoration. This is where Catholics show special honor to consecrated bread and wine, whether through genuflection or other means. It is also a Catholic practice to put a consecrated Communion wafer into a special display case called a “monstrance,” where it can be adored devotionally in a chapel or used in religious processions, particularly during the festival of Corpus Christi.
Other major Roman Catholic dogmas include a belief in the immaculate conception of the Virgin Mary and her bodily Assumption. While all Christians believe Jesus had an immaculate conception--that he was born free from original sin inherited from Adam--Roman Catholics insist on Mary also having the similarly miraculous conception as a point of orthodoxy. Moreover, they also believe her body was assumed--taken up--into heaven at the end of her earthly life. Her corpse is not to be found on earth. Alongside Eastern Orthodox Christians and some Protestants, Catholics believe that Mary remained a perpetual virgin even after Jesus Christ’s birth.
Source:
https://www.christianity.com/church/denominations/what-is-catholicism.html