The Protestant reformation started when Martin Luther posted his 95 Theses (which explained what was wrong with the Catholic Church at the time). People began to see corruption and incorrect translations all for the gains of bishops and the pope. The Protestant reformation crafted the Protestants which branched off. Different branches of Protestant Christianity includes Lutheranism, Baptism, Methodism, etc.
This heavily influenced Christianity in the United States and led to more people becoming Christian after seeing that Protestantism was more relaxed on the rules than Catholics were.