Is suffering from obsessive-compulsive whispers and doubts about God's existence, despite having faith and a desire to remain steadfast in Islam, considered a sin or apostasy?

Whispers (waswas) do not cause harm, nor does one incur sin for them if one dislikes and shuns them. It is obligatory to strive against them, as there is good in doing so. A Muslim must also take the means to achieve certainty by contemplating Allah's signs in the universe, reflecting on the Quran, and consulting a trustworthy doctor, in accordance with the Prophet's (peace be upon him) recommendation.