Question

What part of speech is the word "in" as it is used in this sentence?
"Well, the Romans believed in an anthropocentric universe."

Answer 1

IN is a preposition. It is part of the list of prepositions. Because of that, I believe it is a preposition.

Answer 2

In can be a noun, a preposition, an adverb, or an adjective.
Here I believe it to be an adjective since it is describing the verb believe.