Is it correct to say that God will not accept the repentance of one whom Satan tempted to disbelieve ten times and he succumbed, despite his ability to resist the temptation, relying on God's mercy?

Whoever utters disbelief deliberately has disbelieved, except for the person afflicted with obsessive-compulsive disorder (waswas) who cannot control their speech; they are not held accountable.

If one deliberately utters disbelief, the door to repentance is open for them, based on the Almighty's saying: "Say to those who have disbelieved, if they cease, what has passed will be forgiven for them," and His saying: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'"

To enter Islam, one must pronounce the two testimonies of faith (Shahadatayn) and repent from the apostasy they committed. If they were already uttering the Shahadatayn while in a state of disbelief, it is not sufficient for re-entering Islam unless they acknowledge what they had denied.

A person afflicted with waswas is advised to make supplication, seek treatment, and avoid despairing of Allah's mercy.