As early as 52 A. D. St. Paul wrote an epistle to the Christians of Thessaloniki [Greece], so Christianity has been in Europe for many centuries. However, it has been threatened for many hundreds of years. In 301 A.D. Armenia was the first country to adopt Christianity as its national religion; the Roman Empire followed in 380 A.D. The early part of the Middle Ages witnessed the spread of Christianity in Europe, due in large part to Charlemagne, who conquered the pagan Saxons and forced them upon the penalty of death to embrace Christianity. Charlemagne, who became an emperor of Western Europe, sought to defeat the Moors, who were Muslims, in 778, but could not. The Crusades, begun in 1095 and sanctioned by Pope Urban II, were an attempt to reclaim the Holy Lands and Jerusalem from the Muslims. In the twelfth century the Scandinavian countries were originally converted to Christianity, although many of the old legends and beliefs were hard to dispel. In 1387 the city of Thessaloniki, where European Christianity originally began, was captured by the Turks from the Venetians.
Of course, the Pope was the leader of Christians in Europe with the Papal Seat in Rome and, for a while in Avignon, France, as well as one other city in Europe. The Catholic Pope wielded much power as the king of Italy and spiritual superior of other kings, crowning emperors such as Napoleon. Other religious leaders such as Cardinal Richelieu in France, who "whispered in the ear of kings," were highly influential. With explorations into the New World by the nations of Spain, France, and England Christianity was spread throughout South and Central America and Mexico.
Catholicism spread throughout Europe until the English Reformation when Henry VIII broke from the Catholic Church between 1532 and 1534, when the "Act of Supremacy" established King Henry as the Supreme Head of the Church of England. Between 1517 and 1648 the Protestant Reformation took place.
While Christianity dominated Western Europe, the Ottoman Empire became larger in Southeastern Europe; later, however, in the 16th century the France-Hapsburg rivalry along with other and civil conflicts of the Holy Roman Empire began to distract the Christians from their conflict with the Ottomans and they succeeded in conquering what is now Hungary. By the 17th and 18th century, European advances in weaponry and military tactics became superior to the skills of the Ottomans; consequently, the Hapsburgs were able to retake Hungary and the Great Turkish War ended, restoring Christianity to Europe.
Christianity affected Europe in several ways. One of the main things that come to mind is Roman Catholics, and their Cathedrals. They are truly amazing buildings which give Europe a sense of culture and tradition. It's not modern like for example, Los Angeles California.
Another perk of Christianity that affected Europe is the art and famous paintings.
Look at Europe today, there are all forms of Christianity, whether it's Catholicism or Eastern Orthodox, it does play an important factor. Also, think about how it indirectly affects Europe. During the time of the exploration, many European countries forced Christianity onto slaves and the natives. These choices ultimately affected Europe and other countries. It caused many advantages and disadvantages (such as the inquisition), Christianity molded the ideals and laws that govern Europe today. When addressing the question, you could see it as how it directly affected it, and indirectly (Europe filled with the biggest global powers that impacts other countries that impact them).
Christianity in Europe was a big deal, especially in the 1100's. Catholics were prominent in this time period. The Catholic church was very powerful, and they pretty much controlled the countries in Euorpe during this time period. The Crusades especially affected Europe due to the fact that they had very heavy losses, especially the Children's Crusade, which ended when the children were kidnapped, killed, or sold as slaves. The Pope was considered the most powerful man in the world. If a country did not do what he said or displeased him, they were excommunicated, or kicked out of the church.
Christianty also affected countries by themselves, not just as a whole group. Spain had an entire period in which they considered people who were not Catholic evil, and they slaughtered them in cold blood. Also, King Henry the 8th of England did not particularly like the Catholic church, especially when they refused to let him divorce his first wife. He started the Anglican Church, where he was able to split from his wife. Later, in 1517, Martin Luther saw how corrupted (as he put it) the Catholic church was. They made people pay to be forgiven of their sins, and the sermons were in Latin, which no one understood. Martin Luther believed that people should be able to read from their own bible and not have to pay to have their sins forgiven, so he posted the 95 Thesis on a church door. He is known for starting the Lutheran branch of Protestantism. Soon, the widespread Christianity movement in Europe slowed down a bit after Protestants came into the picture. Catholics and Protestants were constantly fighting and disagreeing with each other, making it more difficult for Christians to come together and spread the Word.
Christianity affected Europe in many ways. The crusades were one of the largest. These wars were fought so Jews could regain Palestine, which at the time was believed by the Muslims that it was rightfully theirs. This debate continues in today's culture. Many Christian kings ruled as well, many changing the regulations for warfare. The most popular are the Peace of God and the Truce of God. The Peace of God forbade lords and counts from attacking certain areas such as monasteries. The Truce of God forbade warfare during special occasions such as Lent of Advent. I hope this helped your studies! If you are looking for a sufficient resource I urge you to either borrow or buy the book Christ the King Lord of History by Anne Carroll:)