We must be guided by the Word of God in order to interpret this situation in the contemporary world and to answer the serious questions it raises.
Starting with Sacred Scripture, we immediately note that there is no mention of “theoretical” atheism, while there is a concern to reject “practical” atheism. The psalmist calls foolish anyone who says in his heart: “There is no God” (Ps 14:1), and behaves accordingly: “They are corrupt, they do abominable deeds, there is none that does good” (ibid.). Another psalm condemns the wicked man who “boasts, ‘He will not avenge it’; ‘There is no God’” (Ps 10:4).
Rather than atheism, the Bible speaks of wickedness and idolatry. Whoever prefers a series of human products, falsely considered divine, living and active, to the true God is wicked and idolatrous. Lengthy prophetic reproaches are devoted to the impotence of idols and likewise of those who make them. With dialectical vehemence, the emptiness and worthlessness of man-made idols is countered with the power of God, the Creator and Wonderworker (cf. Is 44:9-20; Jer 10:1-16).
This doctrine is most fully developed in the Book of Wisdom (cf. Wis 13-15) which presents the way, to be recalled later by St Paul (cf. Rom 1:18-23), to the knowledge of God based on created things. Being an “atheist”, then, means not knowing the true nature of created reality but absolutizing it, and therefore “idolizing” it, instead of considering it a mark of the Creator and the path that leads to him. – from the General Audience, April 14, 1999
