Bless ye God in the congregations - In the assemblages of the people; not only as individuals, but in solemn precessions; in triumphal marches; when the people are assembled together. In this public manner acknowledge God as the true God, and render him praise.
Even the Lord, from the fountain of Israel - Margin, "Ye that are of;" that is, "of the fountain of Israel." The margin has undoubtedly expressed the correct idea. The appeal is to the Hebrew people represented as descending from a common stock or ancestor - Jacob or Israel - as a stream or river flows from a fountain. Compare the notes at Isaiah 48:1; see also Isaiah 51:1; Deuteronomy 33:28. All the descendants of Jacob or Israel are thus called on to unite in solemnly praising the Lord their God.

Bless ye God - This is what they sung.

Bless ye God in the congregations, [even] the Lord, from the fountain (u) of Israel.
(u) Who come of the patriarch Jacob.

Bless ye God in the congregations,.... This was what they said as they sung and played; they called upon the churches, and the members of them, and upon one another, to bless the Lord Jesus Christ, God manifest in the flesh; not by invoking or bestowing a blessing on him, which cannot be, and of which he has no need; but by proclaiming him to be God over all, blessed for evermore, as he is in himself; and the perfections of his nature, by attributing all the blessings of nature, grace, and glory to him, in whom they are, and from whom they come; and by exercising faith upon him for them particularly, and for pardon, righteousness, supplies of grace, and everlasting salvation; and by giving him the glory of all, and ascribing blessing and honour to him on account of them; which should be done openly and publicly, in the several particular congregated churches of Christ; and this shows the psalm still refers to Gospel times, in which only such churches are;
even the Lord, from the fountain of Israel; or, "the Lord, who is of the fountain of Israel" (w); that is, whose natural descent is from Israel, or Jacob, as Christ's was, according to the flesh, Romans 9:5; though some take this to be a description of the posterity of Jacob, those that go out from the fountain of Israel, as Aben Ezra and Kimchi; see Isaiah 48:1; so the Tigurine version, and others; who are called upon to bless the Lord: but then it must be understood not of the carnal Israelites, they rejected the Messiah, Jesus, and called him accursed; but the spiritual seed of Jacob, whether Jews or Gentiles; Israelites indeed, in whom is no guile. The Targum, and so Jarchi, interpret it of the seed of Israel; compare with this Luke 1:41; the words may be read, "for", or "because of the fountain of Israel" (x): God, who is the fountain of living wafers; Christ, whose blood is the fountain opened for sin and uncleanness; the Spirit, who, in the operations of his grace, is a well or fountain of living water, springing up unto eternal life; and all spiritual blessings, and the abundance of them, which the spiritual Israel of God enjoy, may be designed by this phrase; and so be considered as the reason why God the Lord is to be blessed. Some understand it of the Scriptures, from whence all divine knowledge, blessing, and praise are derived; and others of the heart, and the abundance of it, from whence, and not with the lips only, men should bless and praise the Lord.
(w) "qui est ex fonte Israelis", i.e. "natus ex semine Israelis", Tillius & Vitringa apud Michael (x) "Ob vel propter fontem lsrael", Gejerus.

from--or literally, "of"
the fountain of Israel--that is, lineal descendants of Jacob, are invited to unite in the doxology. Then by one of the nearest tribes, one of the most eminent, and two of the most remote, are represented the whole nation of Israel, passing forward (Numbers. 7:1-89).

Fountain - All ye people of Israel.