What is meant by "forgetfulness" in the Hadith, "My Ummah has been forgiven for their mistakes, forgetfulness, and that which they are compelled to do," and does this include someone who commits a sin out of forgetfulness of its prohibition? Does this apply to major sins and acts of apostasy such as reviling religion and polytheism? And what if one commits acts of apostasy out of ignorance?

Allah Almighty has overlooked errors and forgetfulness out of mercy for His servants. This includes everything a Muslim does out of ignorance of the ruling, or forgetfulness, or based on a mistaken interpretation. The imams have agreed that whoever grew up in an environment far from knowledge or was new to Islam, and was ignorant of some rulings, is not to be judged as an infidel until these rulings are made known to him.

There are impediments to religious obligation (taklif): ignorance, forgetfulness, and coercion. So, whoever commits a prohibited act out of ignorance of its prohibition, or out of forgetfulness, incurs no sin. And whoever neglects an obligation due to forgetfulness incurs no sin at the time of forgetfulness, but is obliged to perform it when he remembers it.

All of this applies to matters where ignorance and forgetfulness can be conceived. As for matters where they cannot be conceived, such as insulting religion, for instance, this would not be an excuse. Also, the ignorance that is excused is ignorance of the religious ruling. As for ignorance of the punishment while being aware of the prohibition, this is not an excuse.