Is the doubt in the existence of God—which sometimes accompanies a person despite his conviction in His existence and his remembrance of its proofs—considered disbelief, and how can it be treated?

A Muslim is not held accountable for intrusive thoughts and whisperings (waswas) concerning Allah's essence, for his dislike of them is a sign of faith. He should turn away from them, seek refuge in Allah, and say, "I believe in Allah," or repeat the verse: {He is the First and the Last, the Manifest and the Hidden, and He is Knower of all things} (57:3). He should cease to dwell on them and occupy himself with other matters.