Simplify the following text for a Grade 11 native English-speaking person.
Original: Deism and theism changed meanings slightly in the 17th century , due to the influence of atheism ; deism was originally used as a synonym for today 's theism , but even by its first-found appearance , `` deist '' in 1621 and `` deism '' in 1675 already carried its modern meaning .
Simplified: Deism and theism changed meanings slightly around 1700 - they were influenced by atheism . Deism was originally used as a synonym for today 's theism , but came to denote a separate philosophical doctrine .