Catholic Christianity , as opposed to Arianism and other heretical and schismatic groups , was declared to be the state religion of the Roman Empire on February 27 380 by the decree De Fide Catolica of Emperor Theodosius I. Catholic Christianity , as opposed to Arianism and other ideologies deemed heretical , was declared to be the state religion of the Roman Empire on February 27 , 380 by the decree De Fide Catolica of Emperor Theodosius I.