Thy testimonies [1] are singularly true As yet the Psalmist has insisted upon the excellency of God in the work of creation, and the providential government of the world. Now he speaks of his distinguishing goodness to his chosen people, in making known to them the doctrine which bringeth salvation. He begins by commending the absolute trust-worthiness and truthfulness of the law of God. This being a treasure which was not extended to all nations promiscuously, he adds immediately that the house of God would be adorned with a glory which should last for ever. The Divine goodness is displayed in every part of the world, but the Psalmist justly considers it as of all others the most inestimable blessing, that God should have deposited in his Church the covenant of eternal life, and made his glory principally to shine out of it. Some translate the Hebrew word n'vh, naävah, desirable, [2] as if the Psalmist had said that the adorning of the temple was precious; but the grammatical construction will not admit of this. By length of days is meant perpetual succession, [3] and to this we find Isaiah referring in striking terms, that the Divine truth might be preserved in faithful custody through successive ages. "Behold, I have put my word in thy mouth, in the mouth of thy seed, and of thy seed's seed," (Isaiah 59:21.)

Footnotes

1 - "The testimonies of God, when taken generally, are the truths which he has testified or declared, inclusive not only of moral precepts, but of gracious and unchangeable promises. The combined result of which is, to impress on the minds of men the weighty consideration, that those who trust in the mercy of God must not, in a lower degree, venerate and adore his sanctity in all their converse with him." -- Walford.

2 - "Quidam, n'vh, pro desiderabili accipiunt: acsi dixisset propheta, Templi decus esse pretiosum," etc. -- Lat. The French version follows this exactly. But the sentence is unsatisfactory; and there would seem to be some mistake, or omission, in the original text. If the Hebrew word referred to be rendered desirable, then when joined to qds, the clause would read, holiness is desirable, or becoming, to thy house, etc. -- and not the adorning of thy house is desirable, or precious

3 - "Holiness becometh thine house -- for ever,' l'rk ymym, le-orec yamim, for length of days:' during the whole lapse of time; till the sun and moon shall be no more." -- Dr Adam Clarke

Thy testimonies are very sure - All that thou hast borne witness to; all that thou hast affirmed or declared to be true. This would embrace "all that" God has spoken, whether his law, his promises, his commands, his prophecies, or his statements of what has occurred and of what will occur. See the notes at Psalm 19:7.
Holiness becometh thine house, O Lord - The psalm seems to have been intended to be used in the sanctuary, as a part of public worship, and the word "holiness" here would seem to mean a proper respect for God; confidence in him; a state of mind free from all doubt, and from all that is impure. Perhaps there may be here, also, the idea that in all the convulsions of the world; in all that threatens to overthrow truth and righteousness; in all the attacks which are made on the divine government; in all the efforts of the defenders of error, and in the midst of abounding iniquity, the church should maintain a firm adherence to the principles of "holiness," to that which is right and true. There should be one place - the church - where there would be no wavering in regard to truth and holiness; one place, where the truth would be defended whatever commotions might be abroad. The main idea, therefore, in the psalm is, that, in view of the fact that God reigns, and that nothing can frustrate his plans, or disturb his throne, we should approach him with reverence, with humble trust, with sincere and pure hearts.
In a larger sense, also, in the largest sense conceivable - it is true that "holiness," purity, freedom from evil thoughts, from a wanton eye and a wanton imagination, from unholy plans and purposes, should prevail in the house of God, and should be regarded as indispensable to proper worship. As heaven is pure, and as there shall enter there nothing "that defileth, neither whatsoever worketh abomination or maketh a lie" Revelation 21:27, so in the place where we seek to prepare for that holy world - the sanctuary of God - nothing should be allowed to enter that is impure and polluting; nothing that tends to corrupt or defile the soul. It may be added, that attendance in a place of public worship is calculated to make the heart pure, and to banish unholy thoughts and purposes from the soul. A man who feels that he is in the presence of a holy God, will not be likely to welcome into his soul polluted images and unholy desires.
Forever - Margin, as in Hebrew, "to length of days." The idea is, that it is always appropriate. See the notes at Psalm 23:6.

Thy testimonies are very sure - Thou wilt as surely fulfill thy word as thou wilt keep possession of thy throne.
Holiness becometh thine house - Thy nature is holy, all thy works are holy, and thy word is holy; therefore, thy house - thy Church should be holy. The building itself should be sanctified - should be so consecrated to thy worship alone, that it shall never be employed in any other service. The ministers of this Church should be holy, the members holy, the ordinances holy; its faith, its discipline, and its practice holy. And this at all times, and in all circumstances; for holiness becometh thine house-for ever," לארך ימים le-orech yamim, for length of days. During the whole lapse of time; till the sun and moon shall be no more. The old Psalter says the house of God is man's saule; and of this house holiness is נאוה naavah, "the ornament;" it produces that meek and quiet spirit which is in the sight of God of great price. No decoration of person nor simplicity of dress can supply the place of this heavenly clothing.

Thy (d) testimonies are very sure: holiness becometh thine house, O LORD, for ever.
(d) Besides God's power and wisdom in creating and governing his great mercy also appears in that he has given his people his word and covenant.

Thy testimonies are very sure,.... Meaning not the works of God, of nature, and providence, which testify of his power and Godhead, and other perfections of it; rather the word of God, the Scriptures of truth, the law and Gospel; the one being a testimony of his will to be done by men, the other a testimony of his good will to men; which are sure and true, and to be believed; though it seems best of all to understand them of the promises of God, which testify what he will do, or shall be done hereafter, and which are all yea and amen in Christ; and especially those that respect his kingdom and interest, the glory, stability, and eternity of it, things which are the principal subjects of this psalm; all which promises are sure and certain, true and faithful, firm, and to be believed; see Genesis 19:9,
holiness becometh thine house, O Lord, for ever; or "unto length of days" (d): holiness is a dress or clothing; though it is not the robe of a justifying righteousness, yet it is an inner garment, which makes the saints all glorious within; it is a very beautiful and becoming dress; it is called "the beauty of holiness", and it is always becoming; it becomes the church and people of God, all that are of the household of God, both now and for evermore; it will never be out of use; it will be more and more in use, both in the spiritual reign of Christ, and in the New Jerusalem church state, and in heaven to all eternity; see Zac 14:20.
(d) "in longitudinem dierum", Pagninus, Montanus, Junius & Tremellius, Piscator, &c.

While His power inspires dread, His revealed will should secure our confidence (compare Psalm 19:7; Psalm 25:10), and thus fear and love combined, producing all holy emotions, should distinguish the worship we offer in His house, both earthly and heavenly.

Testimonies - Thy promises, which no less than the precepts are God's testimonies, or the witnesses, or declarations of his will to mankind. He seems here to speak of those precious promises concerning the erection of his kingdom in the world by the Messiah. Holiness - It becometh thy people to be holy in all their approach to thee.