What is the act of copying text or ideas from someone else and claiming them as your own?

I believe the answer is: Plagiarism

Plagiarism is the act of stealing other people's "language, thoughts, ideas, or expressions"  and presenting them as if they're yours.
Using other's people ideas wouldn't be considered as plagiarism if you obtain permission from the original owners, it would fall under fair use.

Iam a german monk and professor of theology. i believe that the catholic church's practice of buying forgiveness of sins through indulgences is wrong. i have translated the bible into german to make it easier for more people to be able to study it. i am here to stand by my writings and i refuse your demand that i recant them