If someone were to memorise the Quran and then forget it, would that be considered a major sin?
Generally the ulama say that it is only a major sin if they forgot out of neglect of the Qur’an when they had the time and opportunity to revise it. As for a person who forgot due to a poor memory, or their memory weakened due to sickness or old age etc, or they were too preoccupied with legitimate occupations and couldn’t keep it as fresh as they should have done, that is not a sin inshaAllah. One should simply try one’s best. The same is true of someone who memorises scattered verses and some of them come and go without any neglect on their part but just the natural ups and downs of memory. For example, they hear someone talking about a verse, and they are reminded of it, even though they hadn’t thought of it for a while.