In Tim Keller’s book Prayer, he stresses the idea that proper prayer comes out of our knowledge of God, which we increase by immersing ourselves in the Bible. It’s our knowledge of who God is that directs how we pray and the types of things we pray about. The greater our knowledge of God, the richer and more varied our prayers.
The Bible not only gives rise to our prayers, it also gives us certainty of God’s response to us. We learn there, in His very words, what He has promised us and how He sees us. Our emotions and impressions, on the other hand, are not reliable indicators of God's response. Therefore, Keller cautions, “[W]e cannot be sure he is speaking to us unless we read it in the Scripture.”
As an illustration of this, Keller tells a story about the great preacher George Whitefield:
If we leave the Bible out, we may plumb our impressions and feelings and imagine God saying various things to us, but how can we be sure we are not self-deceived? The eighteenth-century Anglican clergyman George Whitefield was one of the spearheads of the Great Awakening, a period of massive renewal of interest in Christianity across Western societies and a time of significant church growth. Whitefield was a riveting orator and is considered one of the greatest preachers in church history. In late 1743 his first child, a son, was born to he and his wife, Elizabeth. Whitefield had a strong impression that God was telling him the child would grow up to also be a “preacher of the everlasting Gospel.” In view of this divine assurance, he gave his son the name John, after John the Baptist, whose mother was also named Elizabeth. When John Whitefield was born, George baptized his son before a large crowd and preached a sermon on the great works that God would do through his son. He knew that cynics were sneering at his prophecies, but he ignored them.
Then, at just four months old, his son died suddenly of a seizure. The Whitefields were of course grief-stricken, but George was particularly convicted about how wrong he had been to count his inward impulses and intuitions as being essentially equal to God’s Word. He realized he had led his congregation into the same disillusioning mistake. Whitefield had interpreted his own feelings—his understandable and powerful fatherly pride and joy in his son, and his hopes for him—as God speaking to his heart. Not long afterward, he wrote a wrenching prayer for himself, that God would “render this mistaken parent more cautious, more sober-minded, more experienced in Satan’s devices, and consequently more useful in his future labors to the church of God.”
The lesson here is not that God never guides our thoughts or prompts us to choose wise courses of action, but that we cannot be sure he is speaking to us unless we read it in the Scripture.
If a greater man of God than we will ever be can be mistaken about impressions, then none of us should depend on them. You can read what Greg has written on this subject in Does God Whisper? Part 1, Part 2, and Part 3 or The Ambassador’s Guide to the Voice of God.