Pope Honorius I was condemned as a heretic by three ecumenical councils. All newly elected popes had to profess his condemnation before they could assume their office until the eleventh century and all Latin priests recited it in their breviary until the sixteenth.
Pope Honorius I (625-38) was posthumously condemned as a heretic and excommunicated from the Church by the ecumenical Council of III Constantinople (680-1). He promoted the heresy of the Monothelites, who taught that there is only one will in Christ; the orthodox doctrine is that Christ has separate wills in his human and divine natures.
The council specifically stated that Honorius had advanced heretical teachings, approved of them, and in a positive sense was responsible for disseminating them (and was not merely negligent, as some apologists still lie.) It condemned him by name as a heretic, anathematising him as such and excommunicating him.
He didn’t start up a new religion; however what he did do was bad enough.