Human curiosity and desire for understanding of their environment. Who are they. How did they come to be. What happens when they die. Etc. So humans came up with explanations which evolved from there. As explanations turned into beliefs and differed different humans wanted to dominate in their beliefs over those who differed, less their explanation and understanding of things become unravelled or someone else be right and show them up. Whether true or not, this is one possible explanation for how so many religions came to be, or it could be argued that there is one reason for everything (divine all knowing being) that has always been that humans don't have the capacity to understand the complete origins or workings of and over time different humans misinterprited the divine beings messages to humanity about their questions and desire for explanations or purposefully altered them to suite their individual take on things hence multiple religions.
God created everything in the Old Testament. The people refused to be obedient to God and sinned greatly. So God promised a savior to come and be a sinless man to die and cover our sins. Jesus came and did die and told His disciples to go out and teach and preach. So churches began to spring up. Many people misinturpreted what Jesus taught and branched off into many different beliefs and churches. That continued over a couple thousands years and now we have hundreds of different religions.
Somebody wanted an answer for rain, and stars, so he thought, "it would be hilarious if i tricked people for thousands of years that theres a man in the sky who made the world" and thats how it happend.
It started when the uneducated looked for reason as to the power behind things they couldn't control or explain, planets, stars, the weather, ocean, animals etc. Started of as superstition, became mythology and then cult before religion. http://en.m.wikipedia.org/wiki/Timeline_of_religion
Gen. Chapter 11_ Tower of Babel: A culture that wanted to reach God in their own way, this example even exists today. Trying to reach The Almighty God in your own strength causes religiosity or maybe even a deviation from the truth which is the birth of a "religion." Always keep in mind religion is manmade, JESUS CHRIST IS NOT! MATT 10:39 JESUS SAID, "He that findeth his life shall lose it: and he that loseth his life for my sake shall find it."