The word sacred means “set apart” from the Latin sacer. The Latin word for priest, sacerdos, is also derived from sacer such that the word “priest” could be defined as “one set apart”. In the Old Testament, Israel was a holy nation, a nation that was set apart from other nations (Exodus 19:9). Within the sacred nation, there was also the tribe of Levi, known as Levites, who served as priests and were separated from the other tribes (Numbers 18:20-29). In the New Testament, when Christ instituted his Church upon Peter, he was not abolishing the nation of Israel, but rather, redeeming it and fulfilling its true purpose of sanctifying all nations. Thus, the Catholic Church can be seen as the true continuation of the holy nation of Israel, and her priests as the new Levites.
Now, Catholic priests are priests of Jesus Christ, who is himself the true high priest, continually offering his sacrifice of himself on Calvary to the Heavenly Father in expiation for our sins. Jesus is the sole mediator between God and man because he alone was both God and man; however, priests at their ordination are incorporated into this mediation through the grace of God. Consequently, they become alteri Christi, or other Christs, in the world, and in their sacramental ministry, they operate in persona Christi, or in the person of Christ. After ordination, they become mediators between man and God insofar as they participate in Christ’s mediation between God and man.
However, a priest is not always consumed in his sacramental ministry, of course. On the contrary, priests’ time is often spent on mundane activities such as grocery shopping, teaching, or meetings. In these activities, the priest is not acting in the person of Christ but in his own person. Moreover, there is a huge difference between acting in the person of our Lord Jesus Christ and as a regular human being. Due to the extreme reverence we ought to show the second person of the Blessed Trinity, it is fitting and expedient for us as forgetful humans to remind ourselves via external symbols of his invisible presence. As a result, it is fitting and expedient for the priest to visibly differentiate himself while he is in persona Christi as opposed to when he is not, and this the priest does by wearing liturgical vestments and employing reverent posture.
Nevertheless, it would not be altogether symbolically sufficient if a priest were to only don a liturgical “uniform”, so to speak, for even in the secular sphere different occupations require different uniforms. Consequently, a priest should speak in a sacred language as well.
A priest should speak in a sacred language during sacramental ministry, except for when communication to another person is necessary, both for his benefit and the lay person or persons he is serving. As for the priest, speaking in a sacred language engages a priest’s brain and continually reminds him that he is doing the work of God and not ordinary work. This helps him to be in a more reverent mindset during his sacramental ministry. Moreover, the sacred language has a similar effect on the laity. When the priest speaks in a sacred language, the laity understand that the priest is talking to God rather than talking to them. Also, when a priest uses a sacred language, it signals to the faithful that they are in a sacred place, that they are in a place set apart from the outside world. With the use of a sacred language, not only is the priest better able to differentiate between his sacred and ordinary work but the laity is also better able to discern to whom the priest is talking and the sacredness of what is taking place.
Ultimately, the language used in the liturgy or sacraments should serve as a type of divider between the sacred and the “profane”, or ordinary. The priest should speak in a sacred language as often as possible when speaking to the Holy of Holies and the vernacular dialect when speaking to ordinary men. Just as the outer garments of the priest are changed when he performs his sacramental duties so should his mode of speaking, and perhaps even more so. Moreover, if we truly believe that the priest acts in persona Christi during his sacramental ministry, shouldn’t he do as much as reasonably posssible to represent this incredible albeit invisible reality symbolically? And what external symbol could more clearly remind us that we are not in an ordinary place or, in the case of the priest, performing an ordinary task, than language?