From a commentary on the psalms by Saint Hilary of Poitiers, bishop
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. […] 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 (Psalm 132:2). 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 (Psalm 44:8). […]
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.
Duccio di Buoninsegna, detail from the Maestà Altarpiece of the Duomo di Siena, 1308-11. Museo dell'Opera del Duomo, Siena, Italy.