If the Mass is the Royal Marriage Feast of the Lamb, then the priest should dress up for his entrance into the royal court. The robes should therefore be regal in their dignity, their simplicity and their style. As much as possible their beauty should be shown, not by cleverness of design or ornamentation, but through quality materials and fine workmanship.It struck me that this is just the sort of topic that works well on this blog. Priestly vestments are one of those things that anti-Catholics like to bring up. Just read a little bit on Antichrist Truth and you will see that they quote Revelation 17:4 “the woman was arrayed in PURPLE and SCARLET colour, and decked with GOLD and precious stones and pearls, having a golden cup in her hand” as proof of the Catholic Church being the anti-Christ.
Why should the priest dress like a king? Because he reminds the whole people of God that they serve Christ the King, and the priest is in persona Christi. Furthermore, they remind the people of God that they too are a chosen people, and a royal priesthood. The priest focuses in his own person and ministry the royal priesthood of the people of God. Furthermore, when the priest dresses in fine robes he symbolizes the riches of grace bestowed upon the people of God. Over the black cassock of his sinful human condition the priest wears the white alb--the symbol that he, (and his people) are clothed in the righteousness of Christ by virtue of their baptism. Over that he wears a splendid chasuble to show that the final state of the Christian is not just the white robes of Christ's righteousness, but a share in Christ's own royal priesthood. Each of the faithful are princes and princesses--adopted into the royal family.
In fact, Jesus-Is-Savior points to Baptist ministers wearing liturgical robes as one of several points of ungodly influence of Catholicism on the Baptist churches.
Fr. Longenecker's explanation was great, but if it peaked your interest, here is further reading:
The History and Use of Vestments
Catholic Encyclopedia on Vestments