It is not unreasonable that we should fear God. Jesus Christ Himself says that we are to fear Him who is able to destroy both body and soul in hell. Why? He is the only One who can revoke the judgment of Gehenna fire. The wages of sin is death in Gehenna fire. If we want to escape this punishment, we can see that it is closely connected to whether or not we actually fear God.
Why? What does the fear of God have to do with escaping a judgment that would otherwise take us into the Lake of Fire?
This series of verses in Matthew 10 contains some encouragement, indicating that, if one really fears God, then there is no need to be fearful of others. Proverbs 29:25 plainly tells us, "The fear of man is a snare." This is an attitude in which we do not want to be entrapped. It is obvious, in the context of Matthew 10:27, that He is talking about fear in the sense of "dread." We are not to fear men because the worst that they can do does not even begin to match the worst that God can do! The basis for this is what God is: omnipotent and omniscient, and in Him are the issues of life and death!
The Christian life is our calling; this is our only chance for salvation. We have been personally chosen by God. The elect are an insignificant number, and we are even more insignificant personally. Yet, He has given us this calling. The world population is somewhere in the vicinity of six billion people, and out of this huge number are a miniscule few who are truly converted and have been given the Spirit of God. This is not something that we want to pass up! The fear of God is crucial to our salvation!