Believing in a single god, deity, spirit, etc., especially for an organized religion, faith, or creed.(adjective)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "monotheistic" in a sentence
"The court's reasoning was that the posting of the commandments in the Kentucky courthouses constituted an expression of monotheistic religion but not so in Texas, where the commandments were seen as having more of a secular "educational" purpose."
"And you’d be hard pressed to claim that half the world’s population live in monotheistic cultures even now."
"Sikhism is described as monotheistic unlike the others apart from Islam."