Is it permissible to pray behind an imam who urges congratulating Christians on their holidays and performs acts such as inaugurating a church and visiting places of their celebrations?

Prayer behind this Imam is not permissible. He must be denounced, and people must be warned against him. What he has done is among the great reprehensible acts that are not permissible in the Islamic Sharia. If what this Imam has brought forth is done with his contentment, it is considered among the nullifying transgressions. Scholars have mentioned that prayer behind those who commit nullifying innovations is not permissible.