What is the ruling on saying: "If I were to disbelieve in God, I would become an atheist," when exposed to the whispers of Satan and without having the intention to disbelieve?

The statement is correct in meaning, and there is no harm in saying it, because disbelief (kufr) falls under the general types of atheism (ilhad), which is slandering religion or departing from it, and deviating from the straight path of the Shari'a towards one of the directions of disbelief, or interpreting the essentials of religion to follow desires. The atheist (mulhid) is more general than the zindiq, the hypocrite, and the materialist (dahri); it is the meaning of the disbeliever (kafir) in absolute terms, whether his disbelief was preceded by Islam or not, and whether he openly declared his disbelief or concealed it. It is also advised for those who suffer from obsessive whispers (waswasa) to turn away from them and not to indulge in them.