Here is a quick and simple church history lesson. When the church started the church was not divided as we know it today. Within the first five centuries after Jesus you had three divisions of Christianity that influenced the world: Western-Catholics, Protestants; Eastern: Greek, Orthodox and Copts; and Syriac: Jewish Christianity.
Western Christianity was linked to Rome, used Latin and located in Central Italy, Northern Africa, Southern Spain and Southern France. They were absorbed in Roman culture. They were legal in thought and saw Christianity as a body of laws to obey. Jesus is seen as a legal figure. They were preoccupied with hamnity and human problems and very intereste in sin as a moral response to God. They had simple theology. In the Christ event they emphasize the cross.
Eastern Christianity was lined to Antioch, then Ephesus, then Alexandria and used primarily Greek language. They were located in Aegean islands, northern Egypt, areas Paul preached. They were philisophical, speculative and absorbed in Greek culture. See Christianity as a body of speculative thoght, a philosophy, an explanation of the universe. Jesus is the great philosopher, a teacher, the Word, the giver of life. They were preoccupied with God and His divine action, but not initerested in the topic of sin. Their theology is complex. In the Christ event they emphasize the incarnation.
Syriac (Jewish) Christianity is linked to Edessa and used the Syriac language of Aramaic. Locations were Palestine, mesopotamia, eastern Syria, Egypt, Arabia, and India to China. Asceticism is there offsppring. They were practical, poetic, contemplative and nomadic. They see Christianity as a way of life to live. Jesus is a model or example to be imitated by more spiritual followeres in three ways: Poverty on purpose, Lonely (meaning unmarried) and being a wanderer. Big believers of the succession of Jesus follow the blood line, meaning Jesus' brother James. They used holy writings other than the OT and NT. Considered Christianity to essentailly an ascetic enterprise. Self-discipline is vital.
So you see there is division starting within 500 years after Jesus' return to heaven.
Then you have arguements amongst these groups and within these groups over cities, bishops, how to handle church problems, how to handle so called "traitors", and choices of leadership. This causes further division.
Much later you have Martin Luther who gets the ball moving on the Reformation Movement. He thought the church needed to be reformed because it wasn't correct.
Later again, you have the Restoration Movement also know as the Stone/Campbell Movement, that had the hopes of restoring the church back to the way it was supposed to be in the book of Acts.
There is division because there are humans involved who can't agree and refuse to agree.
Catholicism is considered Christian. It is as many have said before pne of the branches on the Family tree. But, just because anything is considered Christian doesn't mean it truly is. Any church, Catholic, Christain, Baptist, Methodist, Assembly, whatever can have practices within it's walls that are far from Godly and what Christian is supposed to be.
There is your quick and simple history lesson.