John Calvin Commentary Psalms 102:17

"He hath regarded the prayer of the destitute, And hath not despised their prayer." — Psalms 102:17 (ASV)

He has regarded the prayer of the solitary. It is worthy of notice that the deliverance of the chosen tribes is ascribed to the prayers of the faithful. God’s mercy was indeed the sole cause that led Him to deliver His Church, just as He had graciously promised this blessing to her; but to encourage true believers to greater earnestness in prayer, He promises that what He has intended to do of His own good pleasure, He will grant in answer to their requests.

Nor is there any inconsistency between these two truths: that God preserves the Church in the exercise of His free mercy, and that He preserves her in answer to the prayers of His people. For, as their prayers are connected with the free promises, the effect of the former depends entirely upon the latter.

When it is said that the prayers of the solitary were heard, this is not to be understood of one man only (for in the clause immediately following, the plural number is used), but all the Jews, as long as they remained expelled from their own country and lived as exiles in a foreign land, are called solitary. This is because, although the countries of Assyria and Chaldea were remarkably fertile and delightful, these miserable captives, as I have previously observed, wandered there as in a wilderness.

And just as at that time this solitary people obtained favor by sighing, so now when the faithful are scattered and are without their regular assemblies, the Lord will hear their groanings in this desolate dispersion, provided they all in unity, and with sincere faith, earnestly yearn for the restoration of the Church.