Who first used the term alter Christus ("another Christ") to describe an ordained (ministerial) priest?
EDITED TO ADD
Luis Miranda's Manuale Praelatorum Regularium (1612) explicitly calls a priest an alter Christus:
Nam licet Sacerdos secundum se, sit persona finita, non tamen illud offert in sua persona, sed in persona Christi, estque (ut sic loquamur) aequivalenter, quasi alter Christus, in repraesentatione Christi, qui est principalis offerens..."
For although the Priest, in himself, is a finite person, he does not offer it [i.e. the Mass] in his own person, but in the person of Christ, and he is (so to speak) the same as if he were another Christ, in the representation of Christ, who is the principal offerant.
See also An exhortation given by the bishop of Freising (now: Munich and Freising) in 1694, in which the bishop references the fact that St. Francis (not an ordained priest) is often called an "alter Christus" and applies this term to ministerial priests: "...et ita sitis cum Francisco alter Christus."
I also found a book from 1854 with a reference to St. Bernard. The quote from Bernard (1090-1153), which is from De Consideratione, lib. II, c. 8, is close but not quite the same thing. He is speaking about the pope, not just any priest:
Quis es? Sacerdos magnus, summus Pontifex. Tu princeps episcoporum, tu hæres Apostolorum, tu primatu Abel, gubernatu Noe, patriarchatu Abraham, ordine Melchisedech, dignitate Aaron, auctoritate Moyses, judicatu Samuel, potestate Petrus, unctione Christus.
My quick translation:
Who are you? A great priest, the supreme Pontiff. You are the prince of the bishops, the heir of the Apostles. You are Abel by primacy, Noah by governance, Abraham by patriarchy, Melchisedech by order, Aaron by dignity, Moses by authority, Samuel by judgment, Peter by power, and Christ by anointing.
I take it that your question is about the first usage of these words, not this general meaning.
This seems to be surprisingly difficult to track down. Many scholars seem to be content with repeating what others have said without reference, e.g. in Tom Thott, "Sacerdotium - A Reconsideration" (1980):
It was inevitable that the president of the Eucharistic assembly should be associated in a special way with Christ, the one priest. This, incidentally, gives us the only correct understanding of the phrase sacerdos alter Christus, first used by Cyprian, and later taken up by Augustine.
The reference is to Cyprian's Epistle 59 and Augustine's De Civitate Dei XX.9.
I was unable to find anything close to this phrase in Cyprian (though John Paul II also cites Cyprian, but only as the originator of the idea of a Christian as "another Christ").
The Augustine quote (which I find in XX.10, though perhaps that is an issue with editions) is also quite different and never explicitly uses the phrase alter Christus:
Quod autem, cum dixisset: In istis secunda mors non habet potestatem, adiunxit atque ait: Sed erunt sacerdotes Dei et Christi et regnabunt cum eo mille annis: non utique de solis episcopis et presbyteris dictum est, qui proprie iam uocantur in ecclesia sacerdotes; sed sicut omnes christos dicimus propter mysticum chrisma, sic omnes sacerdotes, quoniam membra sunt unius sacerdotis; de quibus apostolus Petrus: Plebs, inquit, sancta, regale sacerdotium. Sane, licet breuiter atque transeunter, insinuauit esse Deum Christum dicendo: Sacerdotes Dei et Christi, hoc est Patris et Filii; quamuis propter formam serui sicut hominis filius, ita etiam sacerdos Christus effectus sit in aeternum secundum ordinem Melchisedech.
English translation from New Advent:
To the words, In them the second death has no power, are added the words, but they shall be priests of God and Christ, and shall reign with Him a thousand years; and this refers not to the bishops alone, and presbyters, who are now specially called priests in the Church; but as we call all believers Christians on account of the mystical chrism, so we call all priests because they are members of the one Priest. Of them the Apostle Peter says, A holy people, a royal priesthood. 1 Peter 2:9 Certainly he implied, though in a passing and incidental way, that Christ is God, saying priests of God and Christ, that is, of the Father and the Son, though it was in His servant-form and as Son of man that Christ was made a Priest for ever after the order of Melchisedec.
The most explicit treatment of this exact issue I can find is in the detailed historical background section of Antonio Aranda, "El cristiano «alter Christus, ipse Christus» en el pensamiento del Beato Josemaría Escrivá de Balaguer" (1994). After giving a review of the basic idea of "configuration with Christ" that is found in Scripture, Aranda gives an excellent review in pg. 525-28 of the patristic basis of calling Christians "christs." The numerous references don't seem to be specific to the ministerial priesthood, however.
Anti-Lutheran controversies led to an increasing emphasis on the ministerial priesthood (as opposed to the universal priesthood of all Christians). Nevertheless, the literal phrase "alter Christus" doesn't seem to appear until the 20th century.
Aranda summarizes his findings on pg. 533-34:
De la expresión recién citada, sacerdos alter Christus, tampoco se conoce su origen inmediato. Cuando la utiliza el magisterio contemporáneo parece tener un sabor tradicional. A decir verdad, en la tradición tampoco está literalmente presente, aunque sí lo esté, como venimos viendo, en cuanto al sentido. Hay que concluir con Gerardi que, no obstante las pistas que algunos señalan, la investigación no ha alcanzado resultados ciertos y seguros. En 1934 el Cardenal Mercier la calificaba de «una especie de adagio teológico», con el que la tradición cristiana expresa su sentimiento sobre el sacerdocio. Lo cierto es que, en esa fecha, quienes la habían utilizado en su literalidad eran principalmente los Papas Pio X, Benedicto XV y Pio XI; antes de ellos no parecen existir referencias ciertas. Quizá tampoco haya que buscar un origen literal inmediato fuera de esas fuentes.
My quick translation:
We also don't know the immediate origin of the phrase just cited, sacerdos alter Christus. When the contemporary magisterium uses it, it appears to have a traditional flavor. In truth, it is also not literally present in the tradition, although it certainly is, as we just saw, with regard to meaning. We must conclude with Gerardi that, despite the paths that some point out, the investigation has not reached any certain and safe results. In 1934, Cardinal Mercier described it as "a kind of theological adage" with which the Christian tradition expresses its feeling about the priesthood. One thing certain is that, at that time, those who had used it word-for-word were principally Popes Pius X, Benedict XV, and Pius XI; before then, there do not appear to be any certain references. Perhaps there is no need to search for an literal, immediate origin beyond those sources.
The earliest reference to Sacerdos alter Christus ("a priest [is] another Christ") I could find is in the proceedings of the Second Plenary Council of Baltimore (1866).
in the new law the Priest ascends to the altar, takes bread and wine, and as another Christ, by the words of consecration converts the their substances into the true body and blood of Christ, created by the lips of the almighty Son of God, indeed, by marvelous and divine power, when He said to his ministers: Do this in my commemoration.
novæ legis Sacerdos ascendit ad altare, sumit panem et vinum, et tanquam alter Christus, verbis consecrationis substantias illas in verum Christi corpus sanguinemque convertit, miranda quidem atque divina virtute a labiis omnipotentis Filii Dei creata, dum ipse suis ministris dixit: Hoc facite in meam commemorationem.
[A priest] is another Christ, vowing himself to the salvation of souls like Christ Himself.
[Sacerdos] est alter Christus, se vovendi saluti animarum sicut Christus ipse.
A Priest of Christ is another Christ. He holds the place of Christ among men, and carries out His duty. As the Father sent me, I also send you. A Priest is selected by Christ and send to continue and perfect Christ's work, which is the salvation of souls.
Sacerdos Christi est alter Christus. Locum Christi inter homines tenet, et vices illius gerit. Sicut misit me Pater, et ego mitto vos. Sacerdos a Christo eligitur et mittitur ad continuandum et perficiendum opus Christi, quod est salus animarum.
Cdl. Henry Manning uses "Sacerdos alter Christus" in his excellent 1870 The Eternal Priesthood (8th ed.):
- Friendship is not only unity of will, but a mutual goodwill each to each. Amicus alter ego. Sacerdos alter Christus. ["A friend is another self. A priest is another Christ."]
These five habits [spirit of penance, study of Holy Scripture, daily prayer, mental prayer, spirit of docility] will continually unfold the seven gifts [of the Holy Ghost] in our intellect and our will, and form in us the habit of mental obedience, the rationabile obsequium ["rational obeisance"], without which a priest cannot be alter Christus, or the likeness of his Master.