The oldest Christian, i.e. biblical use of the term “atheist” comes from the Apostle Paul in Eph. 2:12, and is found in P46—the oldest extant copy of Ephesians (written on papyrus). He used the term to describe the previously pitiable state of the Ephesians, who formerly were “separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God [i.e. ἄθεοι/a-theoi] in the world.” This reminder to the Ephesians served as part of his explanation for how God’s grace reconciles such hopeless people, people like us, to Himself.
This text is Codex Boernerianus, a 9th cent. Greek-Latin interlinear part of my BibleWorks 9 data base. The main text is Greek with a Latin translation above it. Here you can see Eph. 2:4-13 plus the first two words of vs. 14.
Note: in vs. 12, (3rd line from bottom, 3rd word in) you can see both Greek and Latin nomina sacra (abbreviated sacred names) employed for the term atheist. This means that for the Greek, the scribe combined the alpha privative plus the nomen sacrum for God (ΑΘ̅Υ̅), and for the Latin translation also used the nomen sacrum for Deo (dō) above it. Notice also the Latin ligatures, i.e. the use of “&” for et above και.