Verse 17
Lord, thou hast heard - Thou hast not permitted thy tempted and afflicted followers to pray in vain.
Thou wilt prepare their heart - See the economy of the grace of God:
1. God prepares the heart;
2. Suggests the prayer;
3. Hears what is prayed;
4. Answers the petition.
He who has got a cry in his heart after God, may rest assured that that cry proceeded from a Divine preparation, and that an answer will soon arrive. No man ever had a cry in his heart after salvation, but from God. He who continues to cry shall infallibly be heard.