Repentance is a move towards salvation and one's serious decision to stop committing sins is by itself a great victory which opens the gates of divine mercy to the penitent. In fact, he who has reached this stage (repentance) has gained a great victory and he must endeavor to strengthen his motive to combat against sins and to perpetually guard himself against them.

The important point to keep in mind is that one should not despair of divine mercy under any circumstances because the bigger the sin the greater would be the divine mercy.[1] Allah, the Exalted, says:

"Say: O my servants! who have acted extravagantly against their own souls, do not despair of the mercy of Allah; surely Allah forgives the faults altogether; surely He is the Forgiving the Merciful."[2]

Indeed, there are certain conditions for real and genuine repentance. If those conditions are fully observed and materialized, God will accept the repentance. Those conditions are the following: Leaving the sin; remorse over having committed the sin; resolve never to return to the sin; making up for the mistakes and sins he has committed in the past and finally seeking divine forgiveness.[3]

Grand Ayatollah Bahjat's view about your question:

If he is a fitri apostate[4] and he has not declared his apostasy and not caused others to go astray, he must repent and recite the Shahadatain (i.e. the Two Testimonies) once again. He must never make his apostasy known to others. Thus, his body and clothes will be purified and if he is married, he must recite the formula of marriage once again.[5]