Is it possible for someone whose faith weakened, leading him to mock religion, then repented sincerely, to enter the highest Paradise?

We fear that your mockery of religion might merely be a (satanic) whisper, but if it did occur and you then repented with sincere repentance, then repentance erases what preceded it of sin, and you become as one who has not sinned. The Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." Allah the Almighty said: "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.'" So, you must strive in obedience to Allah and draw near to Him, and ask Him to admit you to the highest Paradise, for your sin from which you have repented does not prevent your entry into Paradise, by the grace of Allah.