Tafsir of Al-Mu'minoon 23:99

{حتى} (Until): This relates to [the verb] yaṣifūn (they describe). That is: they remain in their evil speech until this moment.

The verse [98] acts as a parenthetical break between them for the sake of emphasis and to show forbearance toward them, while seeking refuge in Allah from the Shaytan, lest he cause one to slip from patience or incite one to seek vengeance against them. Or, it relates to His saying: {And indeed, they are liars}.

Allah’s address using the plural form is for glorification, as in the poet’s saying: “If you wish, you may forbid the women other than you.” And his saying: “Oh, have mercy on me, O God of Muhammad.”

{إذا} (When): When he is certain of death and perceives the reality of the matter, regret overtakes him for what he neglected regarding faith and righteous deeds therein. So he asks his Lord for a return, saying: {That I might do righteousness} regarding the faith I abandoned. The meaning is: "Perhaps I may bring forth what I neglected of faith and perform righteous deeds therein," just as you say: "Perhaps I may build upon a foundation," meaning: "I shall lay a foundation and build upon it." It is also said: "Regarding the wealth I left behind."

It is narrated from the Prophet (ﷺ): "When the believer sees the angels, they say, 'Shall we return you to the world?' He replies, 'To the house of worries and sorrows? No, rather, I am arriving to Allah.' As for the disbeliever, he says: 'My Lord, send me back.'"

{كلا} (No!): A rebuke against the request for a return, a denial, and a dismissal of its possibility.

{إنها كلمة هو قائلها} (It is but a word he is saying): The "word" refers to the group of words connected to one another, which is his saying: {That I might do righteousness in what I left behind}. He will say it inevitably; he will not abandon it nor remain silent, due to the overwhelming regret and the dominance of remorse upon him. Or, it means he is saying it alone, without being answered or heard.

{ومن ورائهم برزخ} (And behind them is a barrier): The pronoun refers to the group. That is: in front of them is a partition between them and the return until the Day of Resurrection. It does not mean they will return on the Day of Resurrection; rather, it is a total cutting off of hope, for it is known that there is no return on the Day of Resurrection except to the Hereafter.