6 Let the Psalm tell us what they are like. “As the ointment on the head, which descended to the beard, to Aaron's beard, which descended to the fringe of his garment”. What was Aaron? A priest. Who is a priest, except that one Priest, who entered into the Holy of Holies? Who is that priest, save Him, who was at once Victim and Priest? save Him who when he found nothing clean in the world to offer, offered Himself? The ointment is on his head, because Christ is one whole with the Church, but the ointment comes from the head. Our Head is Christ crucified and buried; He rose again, and ascended into heaven; and the Holy Spirit came from the head. Whither? To the beard. The beard signifies the courageous; the beard distinguishes the grown men, the earnest, the active, the vigorous. So that when we describe such, we say, he is a bearded man. Thus that ointment descended first upon the Apostles, descended upon those who bore the first assaults of the world, and therefore the Holy Spirit descended on them. For they who first began to dwell together in unity, suffered persecution, but because the ointment descended to the beard, they suffered, but were not conquered....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)