Catholicism began when Rome entered the fourth century. At this time efforts were underway to hold the Empire together by to uniting all religions..

      April 311 the Emperor Galerius issued an edict of toleration for all religious creeds, including Christianity. Both toleration and restitution had already been granted by Constantine in Gaul, Spain and Britain in 306, and by Maxentius in Italy and Africa in 306 and 310. The Edict, in the form of a joint letter circulated among the governors of the East where various forms of Christianity held sway, declared that the Empire would be neutral with regard to religious worship, officially removing all obstacles to the practice of any religion. It declared unequivocally that the co-authors of the regulations wanted no action taken against the non-Christian groups.

 

      The Emperor Constantine was sympathetic to the religion of his Christian mother, Helena,  But like most Roman emperors, Constantine was a henotheist. Henotheist is devoted to a single God but accepts the existence of other gods. Constantine envisioned a Catholic (universal) church which would reflect these ideas and which would unify the entire empire, which at that time was beginning to fragment and divide.

      Somewhere between 313AD and 325 AD, most of the Christians accepted Constantine's supremacy and accepted his invitation to the Council of Nicea. At this council 50 bishops of the major churches of the Empire began the process of unification. Constantine's financial support of those who worked with him made compromise possible. There were some hard-noses who would not compromise, and they were exiled and their writings burned. 

       From this and subsequent councils Constantine's Roman Catholic Church began to emerge. It was a Catholic Church and was never referred to as Christian until many decades later.

      To please all the factions of the empire and to be a truly "catholic" church elements of all the major religions were combined in its creation. One might say that the Roman Catholic Church  “christianized” the Pagan religions, and “paganized” the Christians. By blurring the differences and erasing the distinctions,  the Roman Catholic Church succeeded in becoming the official religion of the empire.

       Some of the religions that contributed most to the Catholic Church were:

       (1) The Jewish Zealot movement which provided the historical framework for the story.
      (2) Mithraism, a prominent religion in the Roman Empire. ItIt was very popular among the Romans, especially among Roman soldiers, and was possibly the religion of several Roman emperors. Never given “official” status in the Roman empire, Mithraism was the de-facto official religion. One of the key features of Mithraism was a sacramental meal, which involved eating the flesh and drinking the blood of the sacred bull. the god Mithras was “present” in the flesh and blood, and when consumed, granted salvation to those who partook of it. It was called Mazd, which became Mass and was became part of the Church. Mithraism had the original seven “sacraments,” of Roman Catholicism: Anointing of the Sick, Baptism, Communion, Confirmation, Eucharist, the Holy Orders, and Marriage.

       (3) The religion of Isis, an Egyptian mother-goddess religion, was absorbed into Catholicism. Isis has many titles, such as “Queen of Heaven,” “Mother of God,” and “theotokos” (God-bearer). She was given the name Mary out of respect to the ancient Sea Goddess.  Her priestesses were taken from the worship of Juno or Vesta, where they were known as Vestal Virgins. The first clear hints of this comnpromise occur in the writings of Origen, who lived in Alexandria, Egypt, which happened to be the focal point of Isis worship.

      Most Roman emperors (and citizens) were henotheists. Thus the Roman god Jupiter was supreme over the Roman pantheon of gods. Roman sailors were often worshippers of Neptune, the god of the oceans. These secondary gods were later known as saints. There was a Saint Venus, a Saint Mercury and so on. Which is why the Catholic Church has a saint who is “in charge” of these various attributes. The ancient practice of having a god specific to a particular city was continued by the Catholic Church. Only these gods were known as “patron saints.” Jupiter the god of Rome, had his name shortened to Peter and he became the patron saint of Rome and the head of the Roman Church.
      The supremacy of the Bishop of Rome (the Pope) was based on the idea that, since  the Bishop of Rome attended to the Emperor, he was greater than the other bishops. When the Emperor moved his headquarters to Constantinople, the Patriarch of that city made the same claim to eminence. This led to a conflict between the two leaders and eventually into the division of the Catholic church into two: Roman and Greek. 

     The Bishop of Rome acquired more and more power and influence.  When the Roman empire collapsed, the Pope assumed the spiritual title that had previously belonged to the Roman emperors – Pontificus Maximus.

Doctrines
Trinity
Angels
Priesthood
The special authority of the pope
The intercession of the saints
Purgatory as after death place of purification before entering heaven
Transubstantiation - bread  and wine become the true essence of the body and blood of Christ when blessed by a priest in the Eucharist.
Divisions
The Roman Catholic Church, which sees full communion with the Bishop of Rome as an essential element of Catholicism. Its constituent particular churches (Western and Eastern) have distinct and separate jurisdictions, while still being "in union with Rome".with bodies that have maintained the practice. Examples in this category include the Evangelical Lutheran Church in America and the Evangelical Lutheran Church in Canada vis-à-vis their Anglican and Old Catholic counterpa rts