Have you ever heard a voice from above that gave you a powerful message?  Let me share my experience.  When I just a kid, I was out in the hog house doing chores and as I walked out of the building I heard this voice from above that said, “Shake baby shake, I said shake baby shake!”  Since no one else was around, I was a bit inclined to tremble/shake a bit, but then I looked over to where Dad had parked the semi just a few feet away from the hog house.  I then realized that when he had shut it off, he had turned the key too far and the radio was playing a song by Jerry Lee Lewis.  After that there was not much “shaking going on” on my part.
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The subject of voices, dreams, and other forms of extra-biblical revelation often comes up as I minister.  A number of questions come up in this discussion.  One of the first I try to address is, “does God have the power to give extra-biblical revelation?”  My answer is an emphatic, “Yes, God does have the power to reveal anything He wants to any way He wants.”   If God can use a donkey to speak to Balaam then He can use Jerry Lee Lewis to get me to shake, but there is another important question we must consider.  How authoritative would any vision, voice, or dream be? 
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Let me share another of my personal experiences.  I had a dream about rats just before a neighbor came over with his pay loader to bury some junk piles.  Sure enough as he was pushing those piles around, rats came running out.  So was my dream prophetic or was I subconsciously thinking about rats as I slept because I already knew there were rats living in those junk piles that we were getting ready to bury?
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These sorts of questions are real challenges we face as we begin to deal with extra-biblical revelation.  How do we know if this is a specific message from God or something subconsciously manufactured within our own mind?  Let us not forget Jeremiah 17:9, “The heart is deceitful above all things, and desperately wicked: who can know it?”  Furthermore, not only do we have to be concerned about our own hearts deceiving us into thinking something has come from God, we also have to be worried about the devil and his ministers trying to trick us.  “And no marvel; for Satan himself is transformed into an angel of light.  Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.” (2 Corinthians 11:14-15)
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So how do we know that what we are receiving has really come from God?  The answer is actually fairly simple.  Trust the sufficiency of the Bible.  When we go to the Bible, we know that we have God’s word.  2 Timothy 3:16-17 says, “All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:  That the man of God may be perfect, throughly furnished unto all good works.”  Not only are we assured that scripture comes from God, but that it is sufficient to thoroughly furnish us unto all good works.
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Many do not want to give up the idea of extra-biblical revelation because so many of their beliefs are based on things outside of the Bible.  I would argue that most cults and many false doctrines would fall apart if extra-biblical revelation was not embraced.  Let us trust the sufficiency of scripture.