It is good and pleasant for brothers to dwell in unity because when they dwell in unity they are gathered in the community of the Church; when they are called brothers it is because they are of one heart in the charity of a single will.
At the first preaching of the Apostles we read that this was the great precept, summed up in the words, The company of those who believed were of one heart and soul.
It is fitting then, for the people of God to be brothers under one Father, to be one under one Spirit, to enter with one mind under the same roof, under one head to be members of one body.
It is good and pleasant when brothers dwell in unity.
And the Prophet uses a simile to illustrate that goodness and pleasantness, It is like the precious oil upon the head, running down upon the beard of Aaron, running down on the collar of his robes.
By that oil, made up of perfumes, Aaron was anointed to the priesthood. It was God’s pleasure that this be the anointing of his first priest.
And we know that our Lord, too, was invisibly anointed when it is said, Your God has anointed you with the oil of gladness.
That anointing is not a material thing, he was anointed not with a horn of oil as kings are, but with the oil of gladness.
Furthermore, after this anointing Aaron was called according to Law, the Christ, which means the anointed one.
And since wherever it is poured out, this oil quenches the unclean spirits of the heart, so through the anointing of charity we breathe forth to God a sweet odour, as the Apostle says, we are the aroma of Christ.
Just as this oil was pleasing to God in his first priest, so it is good and pleasant for brothers to dwell in unity.
The oil ran down from the head into the beard. Now, a beard is an ornament of adult manhood. For we must not be little children in Christ except, as it is written, we should be babes in evil not in thinking.
The Apostle calls all who have not faith ‘babes’ since they are not strong enough for solid food and still need milk, as he says, I fed you with milk, not solid food; for you were not ready for it; and even yet you are not ready.
Hilary of Poitiers (c.300-368): On Psalm 132; from the Monastic Office of Vigils, Monday of the 6th Week in Ordinary Time Year 2.