Doubting God's love is the root sin behind all other sin. Satan's iniquity began in distrust of God's love for him, and pride, rebellion, and violence followed. Doubt distrusts God's Word, goodness, and power, which amounts to calling Him a liar, and whatever is not of faith is sin (Romans 14:23). God loves each called believer as much as He loves Christ (John 17:23), and the faith Christ seeks in Luke 18:8 is belief in that love. Those who doubt grow cold and fail to endure. Like Peter on the water, believers must look to Jesus rather than to threatening circumstances, remember God's past interventions, and trust that trials are acts of love. Also see sermons and articles about 'God, Doubting'.
Numbers 14:22-23 because all these men who have seen My glory and the signs which I did in Egypt and in the wilderness, and have put Me to the test now these ten times, and have not heeded My voice, they certainly shall not see the land of which I swore to their fathers, nor shall any of those who rejected Me see it.