Religion is about as old as the modern era, first starting in the ancient middle east. Most ancient religions were polytheistic, though some Egyptian kings did believe in one God (and were hated for that reason).
The next popular religion to form was Judaism, somewhere before the start of the Roman empire. these "Hebrews" believed in one God, which was largely unpopular in religion at the time.
After the success of the Roman empire, a religion known as Christianity first made its routes. Most early Christians were blamed for the great roman fire. as such, most of them ended up Martyrs. But, slowly over time, Christianity grew stronger and stronger.
Meanwhile, In the Americas, several empires expanded new religions. These, like the early middle Easterns, were polytheistic.
After the collapse of the roman empire, and the formation of the byzantine empire, Christianity grew. By the Middle Ages, Roman Catholicism was key in the religion world. The church used methods like fear to induce reason and power.
Before the Middle Ages, the religion of Muslims began to grow in the middle east. After large sucess, the Muslims finally got their commitments in "the battle of Tours", where they were pushed back into their territory. The crusades (or holy wars) were a product of religious tensions, mostly between the Christians and Muslims. Many atrocities were committed by the Christians in concurred Muslim's territories, but the harmful behavior surprisingly wasn't traded back when Muslim's finally got the upper hand and won.