What is the ruling on one who forgets what he memorized of the Quran, and is the Quran considered a barrier between him and Paradise if it is not memorized?

Forgetting the Qur'an after memorizing it is a reprehensible negligence of a blessing, and some scholars may consider it a major sin.

More dangerous than forgetting the Qur'an is abandoning its practice and turning away from it, for that is a cause for entering Hellfire.

Allah Almighty said: (And whoever turns away from My remembrance - indeed, he will have a difficult life, and We will gather him on the Day of Resurrection blind) (Taha: 124).

The Prophet, peace and blessings be upon him, said: "The Qur'an is an intercessor whose intercession is accepted, and a truthful disputant. Whoever puts it in front of him, it will lead him to Paradise; and whoever puts it behind his back, it will drive him to Hellfire."

So, the Qur'an intercedes for its companion and leads him to Paradise if he acts upon it, and deprives him of Paradise and drives him to Hellfire if he turns away from it and does not act upon it.

If the pledge to memorize the Qur'an was in the form of "I covenant with Allah" or "I have covenanted with Allah," then it is a vow and an oath that must be fulfilled. If you are unable to fulfill it, you must offer expiation for an oath.

However, if the pledge was in the form of "It is a covenant upon me to memorize the Qur'an," then it is a promise and not a vow, and nothing is required for not fulfilling it. However, it is recommended to fulfill it because it is among the virtuous ethics, especially if the promise is concerning obedience to Allah and memorizing the Qur'an.