About two thousand years ago, Jesus was born to a devout Jewish family in the Roman-occupied and governed territory of Palestine. As an adult, Jesus traveled Israel (Palestine) with a band of disciples, proclaiming the nearness of the kingdom of God, teaching about Scriptures, healing the sick, and forgiving sins. He claimed to be the Son of God. Huge crowds followed him, though he appalled some followers by prophesying that he would die and be raised to life after three days (see Matthew 16:21 and following). Many Jewish religious leaders opposed Jesus, threatened by his claims (and demonstrations) of authority; eventually they succeeded in having him crucified by Roman authorities. Jesus died on the cross and was buried (as he had prophesied); three days later, his disciples found the tomb empty (as he had prophesied). Christians believe the tomb was empty because God had raised Jesus back to life; skeptics believe the tomb was empty because someone stole Jesus' body and duped everyone.
After Jesus' death and resurrection, his disciples spread the good news (gospel) that "God so loved the world that he gave his one and only Son, that whoever believes in him will not perish but have eternal life" (John 3:16) throughout the Roman empire and beyond. Followers of Jesus came to be known as "Little Christs", or Christians (Acts 11:26). Christians believe Jesus to be the Anointed, or Christ (Greek), or Messiah (Hebrew)--the Savior who had been promised long ago in the Jewish Scriptures (which Christians encounter in the Old Testament of the Bible). The earliest Christians were all Jews, and in fact Christianity was originally considered a sect of Judaism, and as such was protected under Roman law as a legitimate religion.
The number of Christians grew, despite persecution, and Christianity eventually became a recognized and established religion. The body of believers, or church, came to have centralized, organized leadership, with the Pope as the very top of the human hierarchy, and ranks of cardinals, bishops, etc. below him. As the church grew, doctrinal disagreements and confusions sometimes sprang up; many creeds, such as the Nicene Creed discussed in this post, were composed in response to these controversies.
After about a thousand years, in 1054, this giant organization split along east-west lines corresponding to the east-west split in the Roman empire, in what is known as the Great Schism. The eastern branch became the Eastern Orthodox Church, and the western branch became the Roman Catholic Church.
Hundreds of years later, the Roman Empire had expired but the (Roman) Catholic Church was still going strong. In fact, it was a powerful political force, with a great deal of wealth. Unfortunately, many people sought positions in the clergy for the sake of wealth and power, rather than for spiritual reasons, with the result that corruption and theological drift were rampant. One of the more egregious problems was the sale of indulgences, a practice which essentially claimed that God's forgiveness of sins could be bought with money.
In 1517, German priest Martin Luther began publicly condemning and protesting the sale of indulgences and other practices of the Catholic church. This was the beginning of the Protestant Reformation, which lead to the birth of the Reformed, or Protestant, Church. Unlike Catholics, Protestants today are split into many different denominations, and have no single central leadership. (Some of these subgroups within Protestantism, in no particular order: Baptists, Evangelicals, Lutherans, Charismatics, Presbyterians, Pentecostals...)
What are the differences in beliefs between these three major branches of Christianity, you may ask? Unfortunately, I know a lot about my own branch of Protestantism but comparatively little about the Catholic church, and next to nothing about the distinguishing doctrines of the Eastern Orthodox Church. I will just write briefly about major differences between Catholics and Protestants, and leave it to you, dear Reader, to learn more by talking to (or reading from) persons who are better informed than I.
What are the differences in doctrine between Catholics and Protestants? Historically, Protestants have focused more than Catholics on personal/individual devotion to God and on reading the Bible for one's self. Protestantism holds that the Bible is the ultimate and only real spiritual authority, whereas Catholicism teaches that tradition is a spiritual authority as well, and that priests and other clergy members have a distinct kind of authority to interpret Scripture and tradition, which unordained believers (laity, or laypeople) do not have. The pitfall of the Protestant perspective is a tendency to excessive individualism, leading to division and uncorrected misinterpretations of Scripture; on the other hand, the pitfall of the Catholic perspective is a tendency to excessive authoritarianism for the clergy and passivity for the laity.
The other two highly visible differences in doctrine stem from this difference in the roles of Scripture vs. clergy. The first is that Protestants give the virgin Mary little attention; she is honored as the mother of Jesus and as a good woman, but not seen as dramatically different from other Christians. Catholics, on the other hand, see Mary's role as far greater; she is important enough to be prayed to. This doesn't mean she is considered to be a second god, though; Catholics sometimes pray to a variety of saints. (Protestants, in contrast, don't even have a process for recognizing saints.) Honestly, I don't understand Mary's role in Catholicism. Ask someone who believes if you want to know more.
The other highly visible doctrinal difference is in the interpretation of Communion. All Christian churches practice the sacrament of Communion (as far as I know), which is a commemoration of the final meal that Jesus shared with his disciples at Passover, on the night that he was betrayed and then crucified. At that Last Supper, Jesus
took bread, gave thanks and broke it, and gave it to them, saying, “This is my body given for you; do this in remembrance of me.” In the same way, after the supper he took the cup, saying, “This cup is the new covenant in my blood, which is poured out for you. (Luke 22:19-20)Christians symbolically re-enact this scene (take Communion) to fulfill Christ's command to "do this in remembrance of me." Catholicism teaches that, in Communion, the bread and wine are miraculously transformed to become, literally, the flesh and blood of Jesus Christ. This doctrine is known as transubstantiation. Protestantism teaches, instead, consubstantiation, the idea that the bread and wine of Communion, while sacred, do not physically become flesh and blood, but symbolically represent the flesh and blood of the Lord Jesus. This difference in interpretation of Communion leads to different practices as to its implementation in Catholic vs. Protestant churches.
Despite these significant differences in doctrine and practice, Catholicism and Protestantism, as well as Eastern Orthodoxy, agree more than they disagree. These three branches of Christianity, flaws and all, are still unified spiritually as the Church, set apart for God. They have Christ in common, and at the end of time all the differences and disagreements will be sorted out in His light.