The devil is a super natural entity found in the bible. If you believe in a super natural entity you are not an atheist. You’re a theist. Theists who believe in (some or all of) the bible are commonly known as Christians.
Mainstream Christians and devil worshipers both believe in the Christian God and the devil. The devil is the foil to the Christian god. God is the good guy, the devil is the bad guy. The devil is an indispensable part of the Christian faith.
The difference is that main stream Christians “worship” the Christian God, while devil worshipers “worship” the Christian devil. If you believe in the devil you’re not an atheist. You’re a Christian.