From a commentary on the psalms by Saint Hilary of Poitiers, bishop
(Ps. 132: PLS 1, 244-245)
Behold, how good and pleasant it is for brothers to dwell in unity! It is good and pleasant for brothers to dwell in unity, because when they do so their association creates the assembly of the Church. The term “brothers” describes the bond of affection arising from their singleness of purpose.
We read that when the apostles first preached, the chief instruction they gave lay in this saying: The hearts and minds of all believers were one. So it is fitting for the people of God to be brothers under one Father, to be united under one Spirit, to live in harmony under one roof, to be limbs of one body.
It is pleasant and good for brothers to dwell in unity. The prophet suggested a comparison for this good and pleasant activity when he said: It is like the ointment on the head which ran down over the beard of Aaron, down upon the collar of his garment. Aaron’s oil was made of the perfumes used to anoint a priest. It was God’s decision that his priest should have his consecration first, and that our Lord should be so anointed, but not visibly, by those who are joined with him. Aaron’s anointing did not belong to this world; it was not done with the horn used for kings, but with the oil of gladness. So afterward Aaron was called the anointed one as the Law prescribed.
When this oil is poured out upon men of unclean heart, it snuffs out their lives, but when it is received as an anointing of love, it exudes the sweet odor of harmony with God. As Paul says, we are the goodly fragrance of Christ. So just as it was pleasing to God when Aaron was anointed priest with this oil, so it is good and pleasant for brothers to dwell in unity.
Now the oil ran down from his head to his beard. A beard adorns a man of mature years. We must not be children before Christ except in the restricted scriptural sense of being children in wickedness but not in our way of thinking. Now Paul calls all who lack faith, children, because they are too weak to take solid food and still need milk. As he says: I fed you with milk rather than the solid food for which you were not yet ready; and you are still not ready.