What is the ruling regarding prayers performed for many years behind an imam who openly disbelieved in God and publicly cursed religion, and sometimes prayed without ablution, given that the congregants were aware of his condition? Do those prayers need to be repeated?

If someone prays behind an imam whose disbelief they know, their prayer is invalid and must be repeated. However, if they learn of his disbelief after the prayer, the majority of scholars hold that it must be repeated, regardless of whether the imam declared his disbelief or concealed it. Some scholars have detailed this, stating that if the imam openly declared his disbelief, the prayer must be repeated, but if he concealed it, it is not obligatory to repeat, and this is a view among the Shafi'is. Ibn Hazm and Al-Uthaymeen are of the opinion that the prayer is valid if the worshiper did not know of the imam's disbelief, because Allah has not burdened people with knowing what is in hearts. In summary, it is preferable and more cautious to repeat prayers for anyone who prayed behind a disbelieving imam, especially if they knew of his disbelief at the time of prayer.