Simplify the following text for a Grade 11 native English-speaking person.
Original: John Calvin 's international influence on the development of the doctrines of the Protestant Reformation began at the age of 25 , when he started work on his first edition of the Institutes of the Christian Religion in 1534 ( published 1536 ) .
Simplified: John Calvin 's international influence on the development of the doctrines of the Protestant Reformation began in 1534 when Calvin was 25 . That marks his start on the first edition of Institutes of the Christian Religion ( published 1536 ) .