When did the Catholic Church begin having celibate priests? Was it a thing which has been since the earliest days or something a later (post 400ad) Pope decided?
WP
The Papal Bull of Pope Siricius in the 380’s mandated priestly celibacy. But although married men were chosen to be deacons, priests, bishops in the early Church, they were required to be continent or be disbarred from the clergy, following the Apostolic example.
There were simple reasons for this: the Twelve were living celibate when they left their families to follow Our Lord and so were living with and like Him. Moreover, the Apostolic Offices of the Twelve was the highest office of the Church, which the Episcopal offices comes closest to but not equal to. The continence required of bishops followed the Twelve’s vocation to live totally for the Kingdom, as Our Lord says in Matthew 19 that some are eunuchs for the sake of the Kingdom.
At first this did not mean separation from one’s wife, but only separation from conjugal sex; this normally would not be a problem, since priests and bishops, and even deacons were chosen from the elders, i.e., those whose children were already raised.
All this is clearly indicated in all the synodal decisions of the Church of the Fathers, East and West, as ably researched and presented by Fr. Cochini, S.J., in his Apostolic Origins of Priestly Celibacy, Ignatius Press, 1990.
Among the evidence adduced by Fr. Cochini up to A.D. 390 are: Ignatius of Antioch, Polycrates of Ephesus, Tertullian, Clement of Alexandria, Hyppolytus of Rome, Origen, the Council of Elvira, First Council of Arles, councils of Ancyra, of Neo-Caesarea, Nicaea, Gahgres, then several Fathers of the Church, etc., etc…
From 390 to the end of the 5th century, besides Roman pontiffs, are numerous councils and synods: Chalcedon, 17th Council of Carthage, 1st Council of Toledo, Council of Orange, 2nd of Arles, 1st of Tours, of Turin; the Armenian Canons of Gregory the Illuminator, Council of Chahabivan, of Seleucia-Ctesiphon, of BethLafath, of Mar Acacius, of Mar Babai; several more from this period.
The 6th century has councils from Africa, Spain and finally the Quinisext synod in Trullo, which is the very first synod of all mentioned previously, including from the Syriac Tradition (which later compromises under Nestorianism) that changes the practice for deacons and priests, but with several restrictions which indicate the reasons for the earlier stricter discipline received from the Apostles: sexual fasting before the Eucharist, no marriageafter ordination (not even for widowers), and celibacy for the Episcopate.
By the 11th century, this discipline broke down in the West due to extreme secularism, so Gregory VII re-established it by restricting candidates to celibates, either single or widowers. The Protestant Reformers revolted against this and Trent re-imposed it, which, because of the seminary system it installed, was able finally to carry it off rather thoroughly for awhile.