So, as my friend Deacon Greg Kandra likes to note, whenever the topic turns to "Roman collars" it invariably progresses to "deacons in collars" and everyone seems to have an opinion about that! During my days at the USCCB, I had occasion to hear each and every argument pro and con on this matter -- repeatedly. I think it's helpful to have some historical perspective on all of this.
2) Where and when did the idea of a distinctive dress for Christian clergy come about? Obviously, early Christians, like St. Peter or St. Paul, or Phoebe, wore no distinctive attire that identified them as "clergy" or even as someone serving others in some kind of ministry. Consider this quotation from Pope Celestine in 428 AD to some bishops in Gaul. After he takes them to task for wearing clothing which made them conspicuous, he writes that "we should be distinguished from the common people by our learning, not by our clothes, by our conduct, not by our dress, by cleanness of mind, not by the care we spend upon our person (Mansi, "Concilia", IV, 465). I would point out that this letter was written after the church had emerged from persecution and, in fact, more than 40 years after the church had become the official and exclusive religion of the empire. It will only be after the adoption of newer clothing styles influenced by the arrival of the Germanic peoples, that bishops decided that clergy should retain the "older" styles, including longer gowns (togas) rather than the shorter "doublets" and "trousers" coming into fashion. The various letters and decrees we see from this time seem to be concerned that clergy dress in a subdued and appropriate manner -- again, without the notion of setting them apart -- that would simply reflect the seriousness of their purpose. We also see the practice of the wearing of "liturgical vestments" (chasubles, dalmatics, e.g.,) as street wear. Finally, even in the later Middle Ages, the concerns of bishops seem to be with the wearing of "closed" capes and full length, somber vesture. This was to counter the practice of many clerics of all clerical ranks to dress more like nobles, with colorful and costly clothes.
This brings up an important point: UNIVERSAL church law says very, very little about the SPECIFICS of what clergy should wear, simply that they should wear something distinctive as determined by the bishops' Conference and/or their own bishops. I mean, in theory, the USCCB could easily say that ALL clergy were to start wearing brown suits with white shirts and red ties. Clerical attire has been so varied in both specific clothing and practice, that perhaps it is useful to remember that what we see now has not always been the case, and it need to be the case in the future.
3) Who wears clerical attire? As the name indicates, "clergy" wear the clothing of clergy. So, how is it that seminarians wear clerical attire when they're not yet clergy? We need to remember that UNTIL 1972, a person became a CLERIC through the rite of TONSURE, long before they were ordained to anything. In the seminary, seminarians "received the soutane" [started to wear the cassock] at about the same time they were tonsured, which took place at sometime between the last 4-6 years of seminary formation. For those not familiar with the "cursus honorum", this means that a young man became a cleric with tonsure; then, he would later be ordained porter, then lector, the exorcist, then acolyte (the minor orders). Then, about a year or so before ordination to the presbyterate, he would be ordained a subdeacon, and eventually, a deacon. So, my point is this: Until 1972, SEMINARIANS IN MAJOR SEMINARY WERE CLERGY, even though they weren't yet presbyters, so it was most appropriate that they be in clergy attire.
4) Since 1972, of course, this is no longer the case, and seminaries have adopted different policies, with some having seminarians in clericals, and in others, only deacons wear them. This is adding a LOT to the current confusion.
SHOULD deacons wear clericals? In my opinion, this is a decision best left to the deacon in question. The reason that the OBLIGATION to wear clerical attire is waived for permanent deacons (canon 288) is because there could be a conflict with the deacon's secular employment. I, for example, was a career Navy officer. Without canon 288, I would have had an obligation to wear clerical attire as well as an obligation to wear my Navy uniform. This is the rationale behind c. 288, not to deny deacons the ability to wear clerical attire in appropriate venues. The policy of the Archdiocese of Washington, DC, is, in my opinion, the best-stated policy I've seen: "If, in the professional judgment of the deacon, the wearing of clerical attire will enhance his ministry, he may do so." This puts the emphasis where it should be: It is not a question of a deacon exercising a "right" to wear something (which could border on clericalism), but rather, a deacon wearing attire that will make him more accessible to the people he serves. For example, I was once taking communion to some folks in a retirement/health facility. I noticed that when I made my rounds wearing "civvies" one woman would never receive communion. She was struggling with senile dementia, and she would look up and when she saw a collar, she'd receive; if she didn't see a collar, she'd refuse. This was not the time for a long doctoral dissertation on why she should receive, regardless of the minister! The poor lady just looked for a simple sign, and from that point on, I wore a collar when visiting that facility.
Hope this is helpful. Comments? Disagreements? Questions?