In 1453 Ottoman Turks, a Muslim tribe from Central Asia conquered Constantinople, finally bringing the Roman empire to an end. They renamed the capital city Istanbul and converted its famed church (the Hagia Sophia) into a mosque (see image below). This act caused shockwaves throughout Europe. The Byzantine Empire had always been the symbol of Christian dominance in Eastern Europe. It had now been conquered by Muslims.
As Muslim forces made their way into eastern Europe, one could argue that they pushed Western Europe into the Atlantic. Western European merchants did not want to purchase Afro-Eurasian goods from hostile Muslim merchants at a high price. So, they sought access to the Indian Ocean world by venturing into the Atlantic and rounding the Cape of Good Hope in Southern Africa.
According to legend, when Vasco DaGama first entered India, he was asked what he wanted. His response was "Spices and Christians." This story may never have actually happened, however, it perfectly illustrates European motives in exploring the worlds oceans.
By about 1000 CE “The Dark Ages” were over. The Crusades and the Mongols had given Western Europeans a taste of Afro-Eurasian luxuries (spices from Southeast Asia, cotton from India, silk and porcelain from China, gold from Africa etc.) Basically, Europeans desired all of the goods that they missed out on during the Post Classical period. As a result explorers, adventurers and corporations alike scrambled to reach the Indian Ocean in order to make a profit.
An army of Christian missionaries followed with a different goal--to convert pagans to Christianity. The Roman Catholic Church was eager to gain more converts to the faith and so the Popes of the early modern period encouraged missionaries to travel to these new lands and convert pagans (non Christians) to Catholicism.