The only remedy and help in his (otherwise) desperate case. This is not spoken as if it were doubtful whether true repentance should obtain pardon, but whether Simon Magus's repentance were true. Repentance is a condition under which God proposeth our pardon and forgiveness, but it is far from being the cause of it.
Related Commentaries of Acts 8:22
Acts 8:22
22 Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee.