What is the method for declaring repentance for one who has committed apostasy by insulting religion, and is it valid to pray behind someone who insults religion and then repeats the prayer to attract them?

Announcing repentance is not a condition for its validity. Many scholars believe that it is better for the repentant person to conceal himself, especially if there is no one to enforce the hadd punishment upon him. Some have held that it is better for a publicly committed sin that the repentant person publicly announce his repentance from it.

Prayer behind someone who cursed the religion is valid if he repents with a known repentance. Prayer behind a disbeliever is not valid. Whoever prays behind someone known to have cursed the religion, his prayer is invalidated, even if his repentance becomes clear after the prayer. This is because it is not permissible to proceed with prayer behind him before being certain of his repentance.

So, if someone prayed behind an imam whose disbelief he knew, then his Islam became apparent, the prayer must be repeated. This is because they were not permitted to follow him before knowing of his Islam. Whoever apostatizes and then embraces Islam should not be prayed behind except after being certain of his religion. Prayer behind someone known to have apostatized is not valid unless he publicly demonstrates his repentance.

And Allah knows best.