We do not need to have the fears we sometimes associate with James 1:6-8. We can take comfort in the knowledge that mind-wandering and normal doubts and fears, while they are undesirable and should be rooted out, are not really what James has in mind. He is warning against double-mindedness. Double-mindedness requires knowing God's law and making a premeditated effort to subvert it and then justify the behavior.
Double-mindedness did not apparently apply to Uzzah, who broke God's law in ignorance or foolishness (II Samuel 6:6-7). However, it does apply to Saul, whom God ordered to destroy the Amalekites totally, but only accomplished 80% of his objectives (I Samuel 15). When confronted with his compromise, Saul makes a whole series of excuses. Excuses and alibis are the defense mechanisms used by double-minded people. If we put sin out of our lives as soon as we find it, or as soon as it is pointed out to us, we do not have to worry about making and remembering excuses or alibis.
Double-mindedness occurs when we harbor a sin and still appear to live God's way. Tares, during their formative (immature) period, look just like wheat, yet mature wheat and tares do not look the same (Matthew 13:30). Over time, the tare is exposed because it does not mature like the wheat. So a double-minded "Christian" will become obvious by his lack of fruit and worldly, hypocritical attitude and behavior. Interestingly, God leaves the tares among the wheat ultimately to benefit the wheat.
A double-minded person cannot have God's Holy Spirit within him (Romans 8:5, 8-9; Galatians 5:16-17). Jesus says we cannot serve two masters because our allegiance will really be to one or the other (Matthew 6:24). One cannot be a double agent with the world and a member of God's church (II Corinthians 6:17-18; I John 2:15-17).
God demands that we choose one way or the other—but not straddle the fence. We cannot have it both ways. Unless in the battle between the spirit and the flesh we throw down the gauntlet in favor of our spiritual selves, we run the risk of being torn to pieces psychologically and emotionally.
Recall Psalm 119:113: "I hate the double-minded, but I love Your law." Notice that the antidote to double-mindedness is yielding to God's law. Wholeness and singleness of purpose are the result of keeping God's law through the power of Christ working in us. As our Lord reminds us in His Sermon on the Mount, "The lamp of the body is the eye. If therefore your eye is good [single, KJV; focused, directed], your whole body will be full of light" (Matthew 6:22).