What is the ruling on one who merely doubts the Last Day?

If doubt is merely a fleeting thought that is repelled, it does not affect the soundness of faith. Anguish felt because of it is a sign of faith. Abu Dawud narrated from Ibn Abbas that doubt is a thought that occurs to everyone, and if a person finds something (of doubt) within himself, he should say: "He is the First and the Last, the Evident and the Immanent, and He is, of all things, Knowing."

However, if doubt is settled and its owner is content with it, then he becomes a disbeliever thereby, because firm faith in the Last Day is a pillar of faith without which faith is not valid, as stated in His (Allah's) saying: "But righteousness is [in] one who believes in Allah and the Last Day and the angels and the Book and the prophets," and in the Hadith (definition) of Gabriel: "To believe in Allah, His angels, His books, His messengers, and the Last Day, and to believe in divine decree, both its good and its evil."