This is a sort of negative promise, but we know that the opposite of it is true: If we do not regard iniquity in our hearts, the Lord will hear us. This text does not refer primarily to some sin that has surprised a person, into which they have fallen by unexpected temptation, but to sin that is regarded, loved, cherished in the heart. This sort of sin blocks the prayer line to heaven.
In Coleridge's "Rime of the Ancient Mariner" we have a picture of how sin hinders prayer. Unrepentant, but in distress, the ancient mariner tried to pray, but could not.
"I looked to heaven, and tried to pray;
But or ever a prayer had gusht,
A wicked whisper came, and made
My heart as dry as dust."
It was only after he repented that he found himself able to pray.
The prophet Isaiah records the warning of God, "When ye spread forth your hands. I will hide mine eyes from you: yea, when ye make many prayers, I will not hear: your hands are full of blood" (Isa. 1:15).
The prayers of those who plan to go on sinning will not be heard, because they "ask amiss" (James 4:3). "We know that God heareth not sinners" were the words of the healed blind man (John 9:31). An unconfessed sin is like a bullet in the body. There can be no help until it is removed. Humans call sin an accident; God calls it an abomination. Sin is a disconnection in the telephone line to glory; it is static in the radio of prayer. Repentance, and repentance alone, will restore communication when the sinner prays the publican's prayer, "God be merciful to me a sinner" (Luke 18:13).
MEDITATION PRAYER: "Wash me throughly from mine iniquity, and cleanse me from my sin" (Ps. 51:2).