In general, any one who exorcises or professes to exorcise demons (cf. Acts 19:13); (2) in particular, one ordained by a bishop for this office, ordination to which is the second of the four minor orders of the Western Church.
The practice of exorcism was not confined to clerics in the early ages, as is clear from Tertullian (Apologet., 23, P.L., I, 410; cf. De Idolat., 11) and Origen (C. Celsum, VII, 4, P.G. 1425). The latter expressly states that even the simplest and rudest of the faithful sometimes cast out demons, by a mere prayer or adjuration (Mark 15:17), and urges the fact as a proof of the power of Christ's grace, and the inability of demons to resist it. In the Eastern Church, a specially ordained order of exorcists (or of acolytes, or door-keepers) has never been established but in the Western Church, these three minor orders with that of lectors as a fourth) were instituted shortly before the middle of the third century. Pope Cornelius (261-252) mentions in his letter to Fabius that there were then in the Roman Church forty-two acolytes, and fifty-two exorcists, readers, and door-keepers (Eusebius, Hist. Eccl., VI, xliii, P.G., XX, 621), and the institution of these orders, and the organization of their functions, seems to have been the work of Cornelius's predecessor, Pope Fabian (236-251).
The fourth Council of Carthage (398), in its seventh canon, prescribes the rite of ordination for exorcist; the bishop is to give him the book containing the formulae of exorcism, saying, "Receive, and commit to memory, and possess the power of imposing hands on energumens, whether baptized or catechumens"; and the same rite has been retained, without change, in the Roman Pontifical down to the present day, except that instead of the ancient Book of Exorcisms, the Pontifical, or Missal, is put into the hands of the ordained. From this form it is clear that one of the chief duties of exorcists was to take part in baptismal exorcism. That catechumens were exorcised every day, for some time before baptism, may be inferred from canon of the same council, which prescribed the daily imposition of hands by the exorcists. A further duty is precribed in canon 92, viz: to supply food to, and in a general way to care for, energumens who habitually frequented the Church. There is no mention of pagan energumens, for the obvious reason that the official ministrations of the Church were not intended for them. But even after the institution of this order, exorcism was not forbidden to the laity, much less to the higher clergy, nor did those who exorcised always use the forms contained in the Book of Exorcisms. Thus the Apostolic Constitutions (VIII, 26; P.G., I, 1122) say expressly that "the exorcist is not ordained", i.e. for the special office of exorcist, but that if anyone possess the charismatic power, he is to be recognized, and if need be, ordained deacon or subdeacon. This is the practice which has survived in the Eastern Orthodox Church.
As an example of the discretion allowed in the West, in the use of the means of exorcising, we may refer to what Sulpitius Severus relates of St. Martin of Tours (Dial., III (II), 6; P.L., XX, 215), that he was in the habit of casting out demons by prayer alone without having recourse to the imposition of hands or the formulae usually employed by the clergy. After a time, as conditions changed in the Church, the office of exorcist, as an independent office, ceased altogether, and was taken over by clerics in major orders, just as the original functions of deacons and subdeacons have with the lapse of time passed to a great extent into the hands of priests; and according to the present discipline of the Catholic Church, it is only priests who are authorized to use the exorcising power conferred by ordination. The change is due to the facts that the catechumenate, with which the office of exorcist was chiefly connected, has ceased, that infant baptism has become the rule, and that with the spread of Christianity and the disappearance of paganism, demonic power has been curtailed, and cases of obsession have become much rarer. It is only Catholic missionaries labouring in pagan lands, where Christianity is not yet dominant, who are likely to meet with fairly frequent cases of possession.
In Christian countries authentic cases of possession sometimes occur and every priest, especially if he be a parish priest, or pastor, is liable to be called upon to perform his duty as exorcist. In doing so, he is to be mindful of the prescriptions of the Roman Ritual and of the laws of provincial or diocesan synods, which for most part require that the bishop should be consulted and his authorization obtained before exorcism is attempted. The chief points of importance in the instructions of the Roman Ritual, prefixed to the rite itself, are as follows:
Possession is not lightly to be taken for granted. Each case is to be carefully examined and great caution to be used in distinguishing genuine possession from certain forms of disease. The priest who undertakes the office should be himself a holy man, of a blameless life, intelligent, courageous, humble, and he should prepare for the work by special acts of devotion and mortification, particularly by prayer and a fasting (Matthew 17:20). He should avoid in the a course of the rite everything that savours of superstition, and should leave the medical aspects of the case to qualified physicians. He should admonish the possessed, in so far as the latter is capable, to dispose himself for the exorcism by prayer, fasting, confession, and communion, and while the rite is in progress to excite within himself a lively faith in God's goodness, and a patient resignation to His holy will. The exorcism should take place in the Church or some other sacred place, if convenient; but if on account of sickness or for other legitimate reasons, it takes place in a private house, witnesses (preferably members of the family) should be present: this is specially enjoined, as a measure of precaution, in case the subject is a woman. All idle and curious questioning of the demon should be avoided, and the prayers and aspirations should be read with great faith, humility, and fervour, and with a consciousness of power and authority. The Blessed Sacrament is not to be brought near the body of the obsessed during exorcism for fear of possible irreverence; but the crucifix, holy water, and, where available, relics of the saints are to be employed. If expulsion of the evil spirit is not obtained at once, the rite should be repeated, if need be, several times. The exorcist should be vested in surplice, and violet stole.