Psalm 66:18 (NKJV)
If I regard iniquity in my heart, The Lord will not hear.
There are an astonishing number of promises in the Bible that God will answer prayer. What a tremendous motivation that is to pray. There are also a handful of verses that indicate that there are hindrances to prayer. One of them is sin. To regard iniquity in my heart means that I know that it is there but I am not willing to repent of it. How could God possibly answer my prayer when I am in such a rebellious state?
Psalm 139:23-24 (NKJV)
Search me, O God, and know my heart; try me, and know my anxieties; and see if there is any wicked way in me, and lead me in the way everlasting.