John Calvin Commentary Psalms 89:17

"For thou art the glory of their strength; And in thy favor our horn shall be exalted." — Psalms 89:17 (ASV)

For you are the glory of their strength. The same sentiment is confirmed when it is declared that God never leaves his faithful servants without strength. The title the glory of their strength, which is attributed to him, means that they are always so sustained by his present help as to have just reason to glory in him; or, what amounts to the same thing, that his power always appears glorious in helping and sustaining them.

They are, however, at the same time, reminded of the duty of giving God all the praise for their being kept safe. If this is true concerning the present life, it is much more truly applicable to the spiritual life of the soul.

Furthermore, to magnify this example of God’s generosity even more, we are taught, at the same time, that it depends entirely on his good pleasure, as there is no other cause for it. From this it follows that they are completely bound and indebted to Him, who is moved by his free goodness alone to continue to extend his help to them.