Christianity is a religion that originated in the Levant after the death of Jesus in 30 AD. The religion was spread by Saint Paul's travels until 60 AD, and it was proclaimed the national religion of the Roman Empire by Constantine the Great in the 300s after years of anti-Christian persecution. The religion was spread through Western Europe due to Rome's vast empire, and was carried to other parts of the world during the Crusades or during the international travels of Jesuit missionaries.
Jesus preached that the goal of Christianity was to treat everyone with respect, the way that they would want to be treated. He said that it would be a belief in the word of God, which was generosity. The religion was popular among the people, as it did not idolize emperors and the idea of "heaven" was where both rich and poor would have a nice afterlife, not the divided heaven that Rome had created.
Jesus Christ spread the first messages of Christianity in the early 1st century AD, with his ideas that he was the son of God diverging from the beliefs of his Jewish elders in Jerusalem. The Jews did not agree with Christ's preachings, and they diverged into Christianity and Judaism. In 37 AD, the Jews conspired with the Romans to kill the "traitor" Jesus, and he was crucified for preaching against Roman paganism. This resulted in animosity between Christians and Jews, who were called "Christ-killers" as a derogatory term.
After the death of Jesus, the apostle (and future first pope) Paul traveled across the Roman Empire to spread the word of Jesus. He traveled from Jerusalem to Rome, everywhere in between, and his letters traveled farther. Under Governor Pliny the Younger of Bithynia, Christians were persecuted in the 70s AD. They would be asked three times if they were Christian; if they insisted all three times they would be punished, but if they changed their mind before three insistences, they would be forgiven. This persecution ended when Roman emperor Constantine the Great claimed to have seen God before the Battle of Milvian Bridge in 312 AD, and he converted to Christianity, the first emperor to do so, and Rome became the first Christian nation.
Christianity remained in the Eastern Roman Empire after the fall of the Pagan and Christian Western Roman Empire, and many Germanic tribes embraced the Christian ideas. The Byzantines, as the Eastern Romans were called after 476 AD, divided from the Papal States in their beliefs, and the Orthodox and Catholicism beliefs split. The Eastern Orthodox church took hold in Russia and Eastern Europe, while Catholicism covered Western Europe. The Catholics obeyed the Pope, while the Orthodox Church had no recognition of him.
Christianity's goodwill ideas were tarnished when the Pope recieved a request for aid against the Saracens by the Byzantine Empire; anticipating that he could rub Catholic influence on the Byzantines, he accepted and invaded the Holy Land. En route, the Jews experienced pogroms from the Crusaders and when Jerusalem fell in 1099, the city's Muslim and Jewish population was butchered. Muslims, Jews, and even Orthodox Christians would be put to the sword until the end of the Crusades in 1270.
After the crusades, Christianity was dormant in Europe, not spreading its word for decades. The Pope and priests held real power in Europe, over the kings and nobles, and they put God's word before proper education that the Greeks and Romans valued. The Muslims kept Greek and Roman learning alive, while the Church dominated civil affairs. This would lead to further splits in the Church in the early Renaissance of the 1500s, with Martin Luther and others forming Protestantism, popular in Northern Europe. Catholicism held on in France and Central Europe, and Christianity was divided into many small groups.
One such group was the Jesuits. These Catholic missionaries, primarily French, Spanish, and Portuguese, traveled across the world and spread their branch of the Catholic faith, which emphasized giving back to society and helping the poor. They even anchored in Japan, Vietnam, and China in the 1500s. The Jesuit faith was embraced by the Shoni and Shimazu in Japan during the Sengoku Jidai of 1545-1615, and it was spread in Vietnam and China as well.
From then on, Christianity of all types was spread across the world, and many African pagans embraced these beliefs after the colonists from Europe took over in the 19th century. Christianity had dozens of different forms, with Roman Catholic being the most popular. It is present in nearly every country, and Islam is the second widest-spread faith. Christianity, Judaism, and Islam are considered "Abrahamic" religions, with the same birthplace, being Jerusalem.