Denying God does not always mean rejecting His existence; often, it involves denying His active involvement in the world. The people adopt a mindset that ignores God and rejects His direct guidance over humanity. The primary approach among commentators is that the people's statements reflect a complete denial of divine providence. They believe God is simply not present, does not monitor their actions, and will neither reward nor punish them [רש״י, מצודת דוד, ביאור שטיינזלץ].
Building on this worldview, the people conclude that the world operates without any system of justice. They assume they can act entirely as they please without facing any consequences [רד״ק, מצודת דוד]. This absence of faith in divine oversight fills them with a false sense of security, leading them to confidently declare that disasters like war and famine will never strike them.
Consequently, the people treat the warnings of God's prophets with complete disregard. Even as the prophets attempt to awaken them with threats of severe punishment, the people brush the warnings aside. They accuse the prophets of lying and insist that no true message has come from God. According to their logic, since God does not see them or know their actions, He certainly would not send prophets to warn them of impending doom [רד״ק].