The dictionary definition of theism is that you believe in the existence of one or several supernatural beings. In this sense Buddha himself is a theist, because the beings in the six realms exist, and enlightened beings exist.
Buddhism is considered atheist or nastika in Indian traditional thought. You can look into the concepts astika and nastika.
An important point is that Buddhism denies that world would be created by Ishvara or some other divinity; self and world are created by Pratitya samutpada, Dependent origination, according to the Dharma.
Buddha teaches that deities cannot do so much for one self as one's own well directed mind can do for one self (Dhammapada).
Some Suttas where Buddha discusses with Devas http://buddhadust.net/dhamma-vinaya/pts ... yc.pts.htm
In Tantra the practitoner/sadhaka is on the same level as the deity or deities, in this sense they are atheist. Certain Indian tantrist thinks that there is tantrism also in Europe, because he has found that some Masons have said that they themselves are the God. Which is the message of Tantra, he says. (Can't remember his name, but he is in Youtube).
Masons are a remnant of the mystery religions that existed in Greece, Rome and Egypt previously. Mystery religions continued in Guilds in Europe for a long time, says Voltaire in his Encyclopedia. "Mystery" translates initiation, roughly.