As the dew of Hermon, and as the dew that descended upon the mountains of Zion: for there the LORD commanded the blessing, even life for evermore.
As the dew of Hermon, (and as the dew) that descended upon the mountains of Zion. As "the precious ointment" (Psalms 133:2) sets forth "how good" is brotherly unity (Psalms 133:1), so "the dew" sets forth "how pleasant" it is. The Hebrew is literally, 'As the dew of Hermon that descended upon the mountains of Zion.' "The dew of Hermon" is an image of "pleasant" or lovely dew. Locally, Hermon in the distant East, was so far from Zion, in the West, that the dew of the former could not literally descend upon the latter. Brotherly unity resembles a "pleasant" dew (such as Hermon has) which descends upon Zion where this unity is so beautifully exemplified (Hengstenberg).
For there the Lord commanded the blessing. "Blessing" is a servant at God's "command" (Leviticus 25:21; Psalms 42:8). He does command its presence on Zion, the representative seat of the kingdom of God, when the people of God are joined "there" in holy brotherhood and "unity." He does not command its presence in the world, and in the region of natural corruption, outside of the kingdom of grace.
(Even) life forevermore. "The dew" is the symbol of ever-fresh youth and resurrection-immortality descending from heaven at Christ's second coming (Psalms 110:3; Isaiah 26:19).