How can one atone for stealing unknown persons' credit card information, and does the ruling differ if they are non-Muslims, given that the value of the stolen amount is unknown?

It is by Allah's grace and mercy that He accepts the repentance of the repentant and forgives those who seek His forgiveness, as Allah Almighty says: "And whoever does a wrong or wrongs himself but then seeks forgiveness of Allah will find Allah Forgiving and Merciful," and His saying, exalted be He: "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," and His saying, عز وجل: "Your Lord has decreed upon Himself mercy: that any of you who does wrong ignorantly then repents after that and corrects himself - indeed, He is Forgiving and Merciful."

The enquirer must return the money he took unlawfully for his repentance to be complete. If he cannot identify the owners, he must give it as charity with the intention that it is on behalf of its rightful owners.