A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 89:1Of Ethan--(See on Psa 88:1, title). This Psalm was composed during some season of great national distress, perhaps Absalom's rebellion. It contrasts the promised prosperity and perpetuity of David's throne (with reference to the great promise of Sa2 7:12-17), with a time when God appeared to have forgotten His covenant. The picture thus drawn may typify the promises and the adversities of Christ's kingdom, and the terms of confiding appeal to God provided appropriate prayers for the divine aid and promised blessing. (Psa. 89:1-52)
mercies--those promised (Isa 55:3; Act 13:34), and--
faithfulness--that is, in fulfilling them.
psa 89:2I have said--expressed, as well as felt, my convictions (Co2 4:13).
psa 89:3The object of this faith expressed in God's words (Sa2 7:11-16).
with--or literally, "to"
my chosen--as the covenant is in the form of a promise.
psa 89:6This is worthy of our belief, for His faithfulness (is praised) by the congregation of saints or holy ones; that is, angels (compare Deu 33:2; Dan 8:13).
sons of the mighty--(compare Psa 29:1). So is He to be admired on earth.
psa 89:8To illustrate His power and faithfulness examples are cited from history. His control of the sea (the most mighty and unstable object in nature), and of Egypt (Psa 87:4), the first great foe of Israel (subjected to utter helplessness from pride and insolence), are specimens. At the same time, the whole frame of nature founded and sustained by Him, Tabor and Hermon for "east and west," and "north and south," together representing the whole world, declare the same truth as to His attributes.
psa 89:12rejoice in thy name--praise Thy perfections by their very existence.
psa 89:15His government of righteousness is served by "mercy" and "truth" as ministers (Psa 85:10-13).
know the joyful sound--understand and appreciate the spiritual blessings symbolized by the feasts to which the people were called by the trumpet (Lev 25:9, &c.).
walk . . . countenance--live in His favor (Psa 4:6; Psa 44:3).
psa 89:16in--or, "by"
thy righteousness--Thy faithful just rule.
of their strength--They shall be adorned as well as protected.
our horn--exalt our power (Psa 75:10; Luk 1:69).
psa 89:18(Margin). Thus is introduced the promise to "our shield," "our king," David.
psa 89:19Then--when the covenant was established, of whose execution the exalted views of God now given furnish assurance.
thou . . . to thy holy one--or godly saint, object of favor (Psa 4:3). Nathan is meant (Sa2 7:17; Ch1 17:3-15).
laid help--literally, "given help." David was chosen and then exalted.
psa 89:20I have found--having sought and then selected him (Sa1 16:1-6).
psa 89:21will protect and sustain (Isa 41:10),
psa 89:22by restraining and conquering his enemies, and performing My gracious purpose of extending his dominion--
psa 89:25hand [and] right hand--power (Psa 17:7; Psa 60:5).
sea, and . . . rivers--limits of his empire (Psa 72:8).
psa 89:26first-born--one who is chief, most beloved or distinguished (Exo 4:22; Col 1:15). In God's sight and purposes he was the first among all monarchs, and specially so in his typical relation to Christ.
psa 89:28This relation is perpetual with David's descendants, as a whole typical in official position of his last greatest descendant. Hence though in personal relations any of them might be faithless and so punished, their typical relation shall continue. His oath confirms His promise, and the most enduring objects of earth and heaven illustrate its perpetual force (Psa 72:5, Psa 72:7, Psa 72:17).
psa 89:35Once--one thing (Psa 27:4).
by my holiness--as a holy God.
that I will not lie--literally, "if I lie"--part of the form of swearing (Sa1 24:6; Sa2 3:35).
psa 89:37It shall . . . moon . . . heaven--literally, "As the moon, and the witness in the sky is sure, that is, the moon."
psa 89:38present a striking contrast to these glowing promises, in mournful evidences of a loss of God's favor.
cast off--and rejected (compare Psa 15:4; Psa 43:2; Psa 44:9).
psa 89:39An insult to the "crown," as of divine origin, was a profanation.
psa 89:40The ruin is depicted under several figures--a vineyard whose broken "hedges," and "strongholds," whose ruins invite spoilers and invaders; a warrior, whose enemies are aided by God, and whose sword's "edge"--literally, "rock" or "strength" (Jos 5:2) is useless; and a youth prematurely old.
psa 89:45days of his youth--or, "youthful vigor," that is, of the royal line, or promised perpetual kingdom, under the figure of a man.
psa 89:46How long, &c.--(Compare Psa 13:1; Psa 88:14; Jer 4:4).
psa 89:47These expostulations are excited in view of the identity of the prosperity of this kingdom with the welfare of all mankind (Gen 22:18; Psa 72:17; Isa 9:7; Isa 11:1-10); for if such is the fate of this chosen royal line.
psa 89:48What man--literally, "strong man--shall live?" and, indeed, have not all men been made in vain, as to glorifying God?
psa 89:49The terms of expostulation are used in view of the actual appearance that God had forsaken His people and forgotten His promise, and the plea for aid is urged in view of the reproaches of His and His people's enemies (compare Isa. 37:17-35).
psa 89:50bear in my bosom--as feeling the affliction of the people (Psa 69:9).
footsteps--ways (Psa 56:6).
psa 89:52Blessed, &c.--denotes returning confidence (Psa 34:1-3).
Amen, and Amen--closes the third book of Psalms.