Tafsir of Al-A'raf 7:51

(Those who took their religion) which Allah the Exalted commanded them [to follow], or [the religion] which they are obligated to profess, (as amusement and play) meaning they did not practice it, or they forbade what they wished and declared lawful what they wished. Amusement (lahw), as has been said, is the turning of one's concern toward that which it is not proper to be turned toward, and play (la'b) is the seeking of joy through that which it is not proper to seek. A detailed discussion regarding these two has already preceded, so be mindful of it.

(And the worldly life deluded them) meaning it occupied them with its fleeting adornments and false promises. Such is its state with those who belong to it—may Allah the Exalted destroy it; it deludes, harms, and passes away.

(So today We will forget them) meaning We will treat them as one who forgets treats the forgotten, by not accounting for them and abandoning them in the Fire entirely. Thus, the statement is expressed in the form of a metaphor. Forgetting (nisyan) has frequently come in the sense of abandonment (tark), and it is valid to interpret it as such here, making it a metaphor or a loose trope. It is reported from Mujahid that he said: The meaning is "We will delay them in the Fire." According to this, it is apparent that "We will forget them" (nansahum) is derived from postponement (al-nas') and not from forgetfulness (al-nisyan). The particle fa (so) in His saying "So today" is fasiha (the 'so' of implication).

His saying "As they forgot the meeting of this Day of theirs" is said to be in the accusative case as an attribute for a deleted verbal noun; that is, "We will forget them a forgetting like their forgetting of the meeting of this great Day, which should not be forgotten." The statement is not in its literal sense either, for they were not mindful of it such that they could forget it; rather, the lack of their bringing the Day of Resurrection to mind and their lack of preparation for it is likened to the state of one who knew something and then forgot it.

It is reported from Ibn Abbas, Mujahid, and al-Hasan that the meaning is "as they forgot to work for the meeting of this Day of theirs." This qualification is not necessary, as is not hidden. Many have held that the ka (as) is for causality, linked to what is implied by the context, not for comparison, since His saying "And they used to reject Our verses" would forbid that, for it is a conjunction to "as they forgot," and that requires it to be that which is being compared. Comparing "forgetting" to "rejection" is not manifest. Whoever claims it says: The meaning is that We will abandon them in the Fire in a continuous abandonment, just as they were deniers that the verses were from Allah the Exalted in a continuous denial.

Al-Qutb said: Rejection is in the sense of forgetting. The apparent meaning of many commentators is that the words of the people of Paradise continue until "And the worldly life deluded them," and that Allah did not forbid it to the disbelievers only. Others said it is that and nothing else, and according to this, it is permissible that "Those" be the subject and the sentence "So today We will forget them" be its predicate, and the fa in it is like the one in your saying, "Whoever comes to me, then he shall have a dirham," as has been said.