Why is the key a Holy Orders symbol?

Answer 1

Although repressed in the Modern rites of ordination, keys were used when a bishop administered the first of the minor orders on a candidate seeking eventual priestly ordination. The keys are proper to the office of Porter. When being ordained a porter, the candidate(s) place their right hand upon the key and the bishop places his hand upon theirs and recites the prayer that confers the order.

A porter used to be responsible for opening and locking places of worship and assuring that the early catechumens (potential converts) left the church when the Mass of the Catechumens had ended (after the sermon) and the Mass of the Faithful was about to begin (with the Creed and Offertory). Porters were also charged with assuring that public sinners were not allowed access to the Church, as well as those who were doing public penance. In short, they were basically a liturgical bouncer. Their practical use began to phase out through the centuries until the office became all but symbolic in most dioceses. Rome finally repressed it along with the other minor orders, those being Porter, Lecter, Exorcist, Acolyte and Subdeacon, in the liturgical reforms of the 1960's. Many Catholic congregations that consider themselves traditionalist have, nonetheless, retained these orders and continue to confer them at the proper times for their candidates to the priesthood calling their repression highly dubious. The absence of being given minor orders does not affect a priestly ordination, for when ordained a priest, the candidate automatically receives all the minor orders in addition to the priesthood. Traditionalists, however, point out that receiving the minor orders at their proper times over the course of priestly training, endows the candidate with the proper graces and dispositions that will aid in his development towards being a good priest.

Answer 2

The key symbolizes the authority given to priests to serve as guides and caretakers of the faith community, unlocking spiritual knowledge and offering access to the sacraments. In Catholic tradition, the keys represent the power to forgive sins as well, echoing the biblical passage where Jesus gave Peter the keys to the kingdom of heaven.