Why did reformation split Christendom?

I'll assume you're talking about luthuranism

Explanation:

The catholic church was basically a huge sellout and Martin Luthor realized it so he wanted to create a new branch of Christianity that was more pure and stuck more stictly to the word of the bible. Catholics didnt like it to much and refused to acknowledge them until 1555 when the Peace of Augsburg forced them to.

At the start of the 16th century, many events led to the Protestant reformation. Clergy abuse caused people to begin criticizing the Catholic Church. The greed and scandalous lives of the clergy had created a split between them and the peasants.