3. Thy people . . . willing--literally, "Thy people (are) free will offerings"; for such is the proper rendering of the word "willing," which is a plural noun, and not an adjective (compare Ex 25:2; Ps 54:6), also a similar form (Jud 5:2-9).
      in the day of thy power--Thy people freely offer themselves (Ro 12:1) in Thy service, enlisting under Thy banner.
      in the beauties of holiness--either as in Ps 29:2, the loveliness of a spiritual worship, of which the temple service, in all its material splendors, was but a type; or more probably, the appearance of the worshippers, who, in this spiritual kingdom, are a nation of kings and priests (1Pe 2:9; Re 1:5), attending this Priest and King, clothed in those eminent graces which the beautiful vestments of the Aaronic priests (Le 16:4) typified. The last very obscure clause--
      from the womb . . . youth--may, according to this view, be thus explained: The word "youth" denotes a period of life distinguished for strength and activity (compare Ec 11:9) --the "dew" is a constant emblem of whatever is refreshing and strengthening (Pr 19:12; Ho 14:5). The Messiah, then, as leading His people, is represented as continually in the vigor of youth, refreshed and strengthened by the early dew of God's grace and Spirit. Thus the phrase corresponds as a member of a parallelism with "the day of thy power" in the first clause. "In the beauties of holiness" belongs to this latter clause, corresponding to "Thy people" in the first, and the colon after "morning" is omitted. Others prefer: Thy youth, or youthful vigor, or body, shall be constantly refreshed by successive accessions of people as dew from the early morning; and this accords with the New Testament idea that the Church is Christ's body (compare Mic 5:7).

JFB.