Will he who is afflicted with obsessive thoughts concerning creed, and asks himself, "What if, after death, we do not find God?" be sinful, despite his complete certainty of the existence of God and the Prophet?

A Muslim is not sinful for what their soul whispers to them, as long as they do not speak of it. The cure for these whispers is to completely ignore them. Turn your mind away from thinking about these matters, and occupy your time with what is beneficial, such as knowledge, righteous deeds, earning a livelihood, exercise, or permissible entertainment. Avoid sitting alone. Be diligent in performing congregational prayers and keeping company with good people, and frequently read the Book of Allah and the books of Hadith and Sunnah. Scriptural texts prove the existence of Allah and the meeting with Him on the Day of Judgment, among them His saying, the Almighty: "And fear Allah and know that you will meet Him. And give good tidings to the believers" (Al-Baqarah: 223).