This is the first of a series of eight psalms Ps. 138-145, placed together in this part of the book, and ascribed to David. They appear to be of the nature of a supplement to the Book of Psalm, composed of psalms unknown to the original collector and arranger of the book, and subsequently discovered and ascertained to be the works of David. It is not to be regarded as strange that there should be psalms of this nature David at different periods which might have been preserved in different branches of his family, and which might not have been generally known to exist. It is rare that the works of an author, especially a poet, are collected and published, and that things of this kind - fugitive and occasional pieces - are not subsequently found; nor is it very unusual that such pieces may, after all, be among the most tender, touching, and beautiful of his compositions. Burns' Highland Mary," so much admired, and his "When wild War's deadly blast was blown," a poem which no one can read without tears - with not a few others of his, are of this description. They are said, in his Biography, to have been "extracted from the correspondence of Burns." (Works of Robert Burns, Philad., 1834, pp. 76, 85, 89.)
The occasion on which this psalm was composed cannot now be determined. It was evidently written in view of trouble Psalm 138:3, Psalm 138:7, and it expresses confidence that God would interpose in the future in behalf of the author, as he had done in the past; and it is, therefore, adapted to inspire confidence and hope in all who are called to pass through scenes of trial. The psalm does not admit of any particular analysis.

The psalmist praises the Lord for his mercies to himself, Psalm 138:1-3. He foretells that the kings of the earth shall worship him, Psalm 138:4, Psalm 138:5. God's condescension to the humble, Psalm 138:6. The psalmist's confidence, Psalm 138:7, Psalm 138:8.
The Hebrew and all the Versions attribute this Psalm to David, and it is supposed to have been made by him when, delivered from all his enemies, he was firmly seated on the throne of Israel. As the Septuagint and Arabic prefix also the names of Haggai and Zechariah, it is probable that it was used by the Jews as a form of thanksgiving for their deliverance from all their enemies, and their ultimate settlement in their own land, after Ahasuerus, supposed by Calmet to be Darius Hystaspes, had married Esther, before which time they were not peaceably settled in their own country.

INTRODUCTION TO PSALM 138
A Psalm of David. This psalm is generally thought to have been written by David upon his being advanced to the throne; on account of which he praises the Lord, who had supported him under many exercises, and had made good his promise to him, at least in part; and he firmly believed the accomplishment of the rest, that he would perfect what concerned him, Psalm 138:8. It seems as if this psalm was composed between his being king over Judah and over all Israel. Though Theodoret understands the psalm as a thanksgiving of the Jews upon their return from Babylon, which David prophesied of. The Syriac version calls it a thanksgiving with a prophecy; as indeed it is a prophecy of the Messiah's kingdom, and of the calling of the Gentiles in the latter day, as appears from Psalm 138:4.

(Psalm 138:1-5) The psalmist praises God for answering prayer.
(Psalm 138:6-8) The Lord's dealing with the humble and the proud.

The Mediator and Perfecter
There will come a time when the praise of Jahve, which according to Psalm 137:3 was obliged to be dumb in the presence of the heathen, will, according to Psalm 138:5, be sung by the kings of the heathen themselves. In the lxx Psalm 137:1-9 side by side with τῷ Δαυίδ also has the inscription Ἱιερεμίου, and Psalm 138:1-8 has Ἀγγαίου καὶ Ζαχαρίου. Perhaps these statements are meant to refer back the existing recension of the text of the respective Psalm to the prophets named (vid., Khler, Haggai, S. 33). From the fact that these names of psalmodists added by the lxx do not come down beyond Malachi, it follows that the Psalm-collection in the mind of the lxx was made not later than in the time of Nehemiah.
The speaker in Psalm 138:1-8, to follow the lofty expectation expressed in Psalm 138:4, is himself a king, and according to the inscription, David. There is, however, nothing to favour his being the author; the Psalm is, in respect for the Davidic Psalm, composed as it were out of the soul of David - an echo of 2 Sam. 7 (1 Chr. 17). The superabundant promise which made the throne of David and of his seed an eternal throne is here gratefully glorified. The Psalm can at any rate be understood, if with Hengstenberg we suppose that it expresses the lofty self-consciousness to which David was raised after victorious battles, when he humbly ascribed the glory to God and resolved to build Him a Temple in place of the tent upon Zion.