If one has forgotten a large portion of the Quran and cannot rememorise it due to weakness of mind and stress, would that person be forgiven by mere tawbah.
In the name of Allah, Most Compassionate, Most Merciful.
Allah states in the Quran, “…Turn to Allah in repentance, all of you, about your past mistakes, so that you may attain salvation.” (Surah Nur: 31)
The door of repentance is open day and night. AbuMusa (radi allahu anhu) narrates that Allah’s Apostle (sallal lahu alayhi wa sallam) said: “Allah, the Exalted and Glorious, stretches out His Hand during the night so that the people repent for the fault committed from dawn till dusk and He stretches out His Hand during the day so that the people may repent for the fault committed from dusk to dawn (He would accept repentance) before the sun rises in the west (before the Day of Resurrection)”. (Sahih Muslim: 1289)
Said bin Jubair narrates that Ibn Abza said to me, “Ask Ibn ‘Abbas regarding the Statement of Allah: ‘And whoever murders a believer intentionally, his recompense is Hell.’ (4.69) And also His Statement: ‘…nor kill such life as Allah has forbidden, except for a just cause …..except those who repent, believe, and do good deeds.’ “ (25.68-70) So I asked Ibn ‘Abbas and he said, “When this (25.68-69) was revealed, the people of Mecca said, “We have invoked other gods with Allah, and we have murdered such lives which Allah has made sacred, and we have committed illegal sexual intercourse. So Allah revealed: ‘Except those who repent, believe, and do good deeds and Allah is
Oft-Forgiving, Most Merciful.’ (25.70)
From the above it is clear that true repentance wipes out all the sins which precede it. If one sincerely repents then Allah would accept this. However for repentance to be sincere and accepted the individual must renounce the sin itself, (in this case make effort to remomerise and also frequent recitation) feel remorse and make a firm determination never to repeat it. The true repentance is characterized by signs, among which are a softness of the heart, frequent weeping and adhering to obedience.
In conclusion if the person makes true effort and keeps on trying and despite this he is unable to remomerise the portion he once knew, then sincerely repentance will inshallah, through Allah’s mercy see his sins forgiven.
Only Allah knows best.
Darul Ifta, Birmingham