Warning of hell fire: Let us focus on two conditions of the heart that are listed first and together. There is a reason that the fearful and unbelieving are listed first and together. Being fearful is an outcome of unbelieving. If we truly believe, then there is no reason to fear. Whenever we experience fear, that would be a sign of our unbelief in God's capability of divine intervention in our lives. Also we must not overlook the word overcometh, where victory is found in submitting to the will of God in our everyday lives. When we do not submit to God's will is when "fearful and unbelieving" will have the preeminence, and the ultimate outcome of that, is hell.
Although there are other behaviors of fear that is apart from the spiritual that God had given us for our own good such as the fear of being run over by a train, if we were standing in front of an oncoming train. Might be a good idea to relocate, therefore also removing the fear. Same with the fear of God, might be a good idea to relocate one allegiance with the world and seek repentance towards God and His loving plan of righteousness. Where there is righteousness, there is grace, peace and love.
Revelation 21:7. He that overcometh shall inherit all things; and I will be his God, and he shall be my son.8. But the fearful, and unbelieving, and abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.