"This is the one to whom I will look: he who is humble and contrite in spirit and trembles at my word." (Isaiah 66:2)
The first mark of the upright heart is that it trembles at the word of the Lord.
Isaiah 66 deals with the problem of some who worship in a way that pleases God and some who worship in a way that doesn't. Verse 3 describes the wicked who bring their sacrifices: "He who slaughters an ox is like one who kills a man." Their sacrifices are an abomination to God — on a par with murder. Why?
In verse 4 God explains: "When I called, no one answered, when I spoke, they did not listen." Their sacrifices were abominations to God because the people were deaf to his voice. But what about those whose prayers God heard? God says in verse 2, "This is the one to whom I will look: he who is humble and contrite in spirit and trembles at my word."
I conclude from this that the first mark of the upright, whose prayers are a delight to God, is that they tremble at God's word. These are the people to whom the Lord will look.
So, the prayer of the upright that delights God comes from a heart that at first feels precarious in the presence of God. It trembles at the hearing of God's word, because it feels so far from God's ideal and so vulnerable to his judgment and so helpless and so sorry for its failings.
This is just what David said in Psalm 51:17, "The sacrifices of God are a broken spirit; a broken and contrite heart, O God, you will not despise." The first thing that makes a prayer acceptable to God is the brokenness and humility of the one who prays. They tremble at his word.
Devotional by John Piper