Simplify the following text for a Grade 18 native English-speaking person.
Original: He belonged to the second generation of the Reformation , publishing his theological tome , the Institutes of the Christian Religion , in 1536 ( later revised ) , and establishing himself as a leader of the Reformed church in Geneva , which became an `` unofficial capital '' of Reformed Christianity in the second half of the sixteenth century .
Simplified: He belonged to the second generation of the Reformation , publishing his theological tome , the Institutes of the Christian Religion , in 1536 ( later revised ) , and establishing himself as a leader of the Reformed church in Geneva , which became an `` unofficial capital '' of Reformed Christianity in the second half of the 16th century .