For thou art the glory of their strength. The same sentiment is confirmed when it is declared, that God never leaves his faithful servants destitute of strength. By the appellation the glory of their strength, which is ascribed to him, is meant that they are always so sustained by his present aid as to have just ground to glory in him; or which amounts to the same thing, that his power appears always glorious in aiding and sustaining them. They are, however, at the same time, reminded of the duty of yielding to God all the praise of their being preserved in safety. If this is true as to the present life, it is much more truly applicable to the spiritual life of the soul. Farther, the more highly to magnify this instance of God's liberality, we are taught, at the same time, that it depends entirely upon his good pleasure, there being no other cause of it. [1] Whence it follows, that they are wholly bound and indebted to Him who is induced by his free bounty alone to continue to extend to them his help.

Footnotes

1 - "The Hebrew lyhvh, must be rendered of or from the Lord, in both places in this verse: Of the Lord is our shield or defense;' Of the Lord, or from him,' i e., of his appointment, is our King.'"-- Hammond

For thou art the glory of their strength - The ornament; the beauty; the honor; that is, Their strength derives its beauty and honor, not from anything in themselves, but from the fact that it is derived from thee. The strength thus imparted is an honor or ornament in itself; it is an honor and glory to them that it is imparted to them.
And in thy favor - Or, by thy favor, or good pleasure.
Our horn shall be exalted - The horn is a symbol of power. Compare Psalm 22:21, note; Psalm 75:4; Daniel 7:8, note; Job 16:15, note. The meaning here is, that their power had been derived from God; or that all which contributed to their exaltation and honor in the world, had been derived from him.

For thou art the glory of their strength - They are strong in faith, and give glory to thee, because they know that their strength cometh from the Lord of hosts.
And in thy favor our horn shall be exalted - Instead of תרום tarum, "shall be exalted," תרים tarim, "thou shalt exalt," is the reading of several MSS.: but תרום tarum, "shall be exalted," is supported by forty-four of Kennicott's MSS., and sixty of De Rossi's, as well as by several ancient editions, with the Septuagint, Syriac, Vulgate, and Arabic Versions. In the enjoyment of the Divine favor they shall grow more wise, more holy, more powerful, and, consequently, more happy.

For thou [art] the (o) glory of their strength: and in thy favour our horn shall be exalted.
(o) In that they are preserved and continue, they should give the praise and glory to you alone.

For thou art the glory of their strength,.... By which they walk, and do all they do, exercise every grace, and discharge their duty; they have their strength from Christ, as well as their righteousness, without whom they can do nothing, but all things through him strengthening them; and as his righteousness exalts them, his strength adorns and glorifies them; how glorious and beautiful does a believer look, that is strong in the Lord, and in the power of his might, in the grace that is in Christ, and in the exercise of faith on him, giving glory to God; on whom the power of Christ rests, and it overshadows, and in whose weakness his strength is made perfect!
and in thy favour our horn shall be exalted; either Christ, the Horn of their salvation, who in an acceptable time, in the time of God's favour, or good will, was heard and helped by him as man, carried through his sufferings and death, was raised from the dead, and exalted at his right hand; see Psalm 89:24 or the saints themselves, their power and strength, kingdom and glory; by the special favour of God in Christ, their mountain is made so strong, and they so highly exalted, as that they think they shall never be moved; and in the latter day the mountain of the Lord's house shall be exalted above the hills, Psalm 30:6.

The glory - To thee alone belongs the glory of all their valiant achievements.