Each of the internal "books" or original anthologies within the book of Psalms concludes with its own doxology. Psalm 72, at the end of Book II, features a doxology at verses 18 and 19. That biblical text is paraphrased in this hymn, a paraphrase originally published in the 1650 edition of the Scottish Psalter. However, what was originally a standard four-line common-meter text has been stretched into common meter of six lines to suit the tune.
"Now Blessed Be" is cast in the berakah form common in Jewish worship. God is to be blessed, or praised, because of his wondrous works and mighty deeds (st. 1) and because his saving acts reveal his glory throughout the earth and throughout eternity (st. 2). Amen and Amen! So shall it be!
Bert Polman, Psalter Hymnal Handbook
How can the worshiper not conclude with such acclamations! When God is the “overflowing source of all good” (Belgic Confession, Article 1) and when he has provided all the benefits of Christ’s atonement and makes them ours so that “they are more than enough to absolve us from our sins,” (Belgic Confession, Article 22) our hearts cry out to him with praise and adoration. Therefore, Heidelberg Catechism, Lord’s Day 52, Question and Answer 128 includes the ending doxology of the Lord’s Prayer and teaches that “your holy name, and not we ourselves, should receive all the praise, forever.” And so consistent with these thoughts, Our World Belongs to God, paragraph 2 exclaims, “Our World Belongs to God! God is King: Let the earth be glad! Christ is victor: His rule has begun! The Spirit is at work: Creation is renewed! Hallelujah! Praise the Lord!” And the Belhar Confession, Section 5 concludes: “Jesus is Lord. To the one and only God, Father, Son and Holy Spirit, be the honor and the glory forever and ever.”