The Greek adjective atheos is formed from a- (without) + theos (G2316, God). It appears only once in the New Testament (Ephesians 2:12) but is theologically explosive. Paul describes the pre-conversion state of the Gentiles: they were without God in the world. Interestingly, early Christians were themselves called atheoi by Romans for refusing to worship the pagan gods — yet Paul reclaims the word to describe the spiritually destitute state apart from Christ.
Atheos describes not merely intellectual atheism but the existential state of being cut off from the source of all life, meaning, and hope. Paul's description of the Gentiles in Ephesians 2:12 is devastating: separate from Christ, excluded from citizenship in Israel, foreigners to the covenants of the promise, without hope and without God in the world. But the very next verse is the hinge of the entire letter: But now in Christ Jesus you who once were far away have been brought near by the blood of Christ (Ephesians 2:13). The Gospel transforms the atheos into a child of God — those once without God are now in God.