Simple Definition:

Calvinism is a religious belief system that follows the teachings of the theologian [a person who studies religion and God] John Calvin.Calvinists believe in the idea of predestination, which means that God has already determined who will go to heaven and who will go to hell. They also believe in the total depravity [moral corruption] of humanity, meaning that humans are inherently sinful and incapable of doing good without God’s grace.

Very Simple Definition:

Calvinism is a religious belief that says God has already decided who will go to heaven and who will go to hell. Calvinists believe that humans are naturally bad and need God’s help to do good things.