Introduction
There does not seem to have been any distinctive everyday dress for Christian pastors up until the 6th century or so. Clergy simply wore what was common, yet muted, modest, and tasteful, in keeping with their office. In time, however, the dress of pastors remained rather conservative, as it is wont to do, while the dress of lay people changed more rapidly. The result was that the dress of Christian pastors became distinct from the laity and thus that clothing began to be invested (no pun intended) with meaning.
Skipping ahead, due to the increasing acceptance of lay scholars in the new universities, the Fourth Lateran council (1215) mandated a distinctive dress for clergy so that they could be distinguished when about town. This attire became known as the vestis talaris or the cassock. Lay academics would wear an open front robe with a lirripium or hood. It is interesting to note that both modern day academic and clerical garb stems from the same Medieval origin.
Councils of the Roman Catholic church after the time of the Reformation stipulated that the common everyday attire for priests should be the cassock. Up until the middle of the 20th century, this was the common street clothes attire for Roman Catholic priests. The origin of the clerical collar does not stem from the attire of Roman priests. Its genesis is of Protestant origin.
The Origin of Reformed Clerical Dress
In the time of the Reformation, many of the Reformed wanted to distance themselves from what was perceived as Roman clerical attire. Thus many of the clergy took up the attire of academics in their daily dress or wore no distinctive clothing whatsoever. Yet over time the desire for the clergy to wear a distinctive uniform returned to the Reformed churches. What they began to do, beginning in the 17th century as far as I can tell, is to begin to wear a neck scarf, called a cravat, tied around the neck to resemble a yoke. Thus common dignified attire was worn by the pastor, supplementing it with this clerical cravat. This style can be seen in many of our famous Reformed divines, one of the more famous of whom being Charles Hodge.