Try the dictionary. That explains it really well
ag·nos·tic
aɡˈnästik/
noun
noun: agnostic; plural noun: agnostics
1.
a person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena; a person who claims neither faith nor disbelief in God.
synonyms: skeptic, doubter, doubting Thomas, cynic; More
unbeliever, nonbeliever, rationalist;
rarenullifidian
"as far as I know, Stevens was an atheist, or at least an agnostic"
antonyms: believer, theist