I suspect that the first-millennium Fathers, had they been forced to express themselves in the terms of Scholastic categories, might have said that the Form of Confirmation was the Prayer for the Spirit which precedes the consignations, or, perhaps, the words within it Emitte Spiritum Sanctum etc.. Then the Pontiff, or his presbyters, did the Matter by consignating the candidates, saying nothing. Compare the pattern of the Roman Rite of Presbyteral Ordination.
It is around the end of the First Millennium that we encounter the Formula
SIGNO TE SIGNO CRUCIS ET CONFIRMO TE CHRISMATE SALUTIS.
It may, of course, have been used long before then. Liturgical books produced for the use of bishops may not always record customs which had grown up but which did not affect the pontiff himself. But this formula, whatever its age, seems to me to have a very great deal to be said for it. Not least because of the strength of the first four words. (Cranmer, incidentally, preserved them in his first, 1549, Book of Common Prayer; and they were restored by the ('usager') Non-Jurors in their Book of 1719.)
Marking ones possessions ... even ones human possessions ... was a convention in the Greco-Roman world. One branded cattle ... and one also tattooed slaves and soldiers who were enrolled under the princeps. A runaway slave was branded HFE (Hic fugitivus est) so that, if he ran away again, he would be cruelly identifiable. It was natural, therefore, for those who followed the Mystery Cults of the early centuries to set a marker of their religious allegiance upon their bodies. Circumcision, of course, is another example of similar thinking; and the Marking of the doorposts of the Jews to avert the Avenging Angel served a similar purpose.
Jewish apocalyptic literature was very familiar with the idea that, in the perils and conflicts of the Last Times, the Righteous would be protected from destruction by having God's Mark upon them. We already find this in Ezekiel 9, where the Scribe is to go through Jerusalem and to mark with the letter Tau those who are still faithful to YHWH. In early Hebrew scripts, Tau was written like a chi (X); or as T or +. Margaret Barker tells us that the mark X is the ancient sign of the Name of YHWH and was marked on the forehead of a High Priest when he was anointed.
(The S Paul VI rite of Confirmation, like a number of post-Conciliar novelties, makes the SIGNUM CRUCIS verbally invisible. It seems almost to be designed to rupture the continuities which link and illuminate the 'Testaments'. Antisemitism at work??)
The customary 'little slap' upon the cheek of the confirmand, by a historical chance, reinforces the valuable notion of Confirmation as the Sacrament of those about to go into battle.
In Antiquity, you might authenticate a letter by (licking your signet ring and then) pressing your Mark into the wax or clay. If you have purchased some merchandise and left it on the quay until you can have it moved into your warehouses, your seal marked all over it will secure it as your possession.
The SIGNUM CRUCIS shows you as God's possession, under his protection, destined, at the eschaton, to be moved into his property.
CONFIRMO (bebaio) suggests not only the legal confirmation of a document but the strengthening necessary in the struggles of the Last Times. CHRISMATE reminds the hearer that we are all Christoi, sharing the Lord's status as we have been anointed with his Spirit. Its physical precision reminds both recipient and Congregation, in words, of what might not be visually accessible. And SALUTIS, triumphantly, indicates the redemptive finality of the rite: Ephesians 1:14 and 4:30 are strongly in evidence here. S Paul pictures the Holy Spirit deposited within us as a pledge given by God, who is faithful to his promises.
I hope nobody will think that I am in any way disparaging the Byzantine formula which the roches of this world desire us now exclusively to use. My sole motive is to make clear that the Western Rite of Confirmation which we have inherited is something of which we have no need ... no need whatsoever ... to be ashamed.
ENDNOTE: the Roman Rite is worth preserving. In this particular matter, it is best preserved by the confirmandi receiving the Sacrament from their bishop, thus expressing and strengthening the communio between them. It will be a tragedy if Bishops and laics alike are deprived of this mark of communio. After all, Canon 87 does give bishops the right to dispense from Traditionis custodes.
But, of course, in these strange and menacing times of clear Necessity, it may be necessary for unusual provisions to be made to secure the survival of this important part of our Latin patrimony ... until the black clouds have passed over.
Former Anglicans will remember Eric Mascall's "[the Bishop] will not come and visit me or take my confirmations/ Colonial prelates I employ from far-off mission-stations."