The First Ecumenical Council:  The First Council of Nice 9Canon VIII.

Concerning those who call themselves Cathari, if they come over to the Catholic and Apostolic Church, the great and holy Synod decrees that they who are ordained shall continue as they are in the clergy. But it is before all things necessary that they should profess in writing that they will observe and follow the dogmas of the Catholic and Apostolic Church; in particular that they will communicate with persons who have been twice married, and with those who having lapsed in persecution have had a period [of penance] laid upon them, and a time [of restoration] fixed so that in all things they will follow the dogmas of the Catholic Church. Wheresoever, then, whether in villages or in cities, all of the ordained are found to be of these only, let them remain in the clergy, and in the same rank in which they are found. But if they come over where there is a bishop or presbyter of the Catholic Church, it is manifest that the Bishop of the Church must have the bishop's dignity; and he who was named bishop by those who are called Cathari shall have the rank of presbyter, unless it shall seem fit to the Bishop to admit him to partake in the honour of the title. Or, if this should not be satisfactory, then shall the bishop provide for him a place as Chorepiscopus, or presbyter, in order that he may be evidently seen to be of the clergy, and that there may not be two bishops in the city.