What should one who is afflicted with doubt regarding faith in the Last Day do?

Whoever doubts his faith should seek refuge with Allah from Satan and frequently recite: "He is the First and the Last, the Manifest and the Hidden, and He is, of all things, knowing." If the doubt is merely a passing thought that a person pushes away, feels distressed by, and dislikes speaking about, then there is no sin in it. However, if his heart becomes tranquil with it and he sees no harm in speaking about it, then that is dangerous, and he must promptly repent.