If no one knows a person practised this sin, and they realize and come back to the Islamic life, then how can they get rescued by Allah? I again mention that nobody knows except the wrongdoers.
"Shirk is the only sin that Allah does not forgive. He may forgive whatever He wills, other than this. For whoever associates partners with Allah does infact go far astray into deviation".
At the time of Judgement, it will be God's will to forgive or not