"Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life." (John 8:12)
I think one could safely say that the distance between the saved individual and the lost man or woman can be measured in light years (1 light year = about 5.88 trillion miles).
The first two chapters of 1 John make this stark distinction quite plain. The difference between being a child of God, by faith in Jesus Christ, and being a child of the devil, but rejection of God's Son, is the difference between light and darkness. God's children may experience temporary darkness, as you might if you were rendered unconscious, but it is virtually impossible for someone who is filled with the "Light of the World" to remain in prolonged darkness.
What, you may wonder, could bring this temporary darkness into the life of a child of God? Sin, in general, is an obvious answer; but 1 John 2:10-11 provides us with a specific one:
"He that loveth his brother abideth in the light, and there is none occasion of stumbling in him. But he that hateth his brother is in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes."
Hate will turn the light out every time, says the apostle. According to the old saying, "Love is blind," but I would contend that love is not blind; it only chooses not to see, at times. On the contrary, it is hate, according to this verse that can be so blinding that we are virtually unable to see where we're going ("...knoweth not whither he goeth..."). Sometimes we piously insist that we don't actually hate, we just strongly dislike a brother or sister in the Lord; but I am very much afraid, the line between the two can become so blurred that we stumble over the line from one to the other, without being conscious of it. After all, that's the kind of thing that happens when you're blind.
David said in Psalm 139 that he was able to hate the enemies of God with a "perfect hatred," no small claim. But he did not include fellow believers in this category. These verses in 1 John are spoken about those among us who allow hatred for a brother or sister in Christ to worm its way into a regenerated heart. For that reason, it would be wise for all of us to guard against strong and/or prolonged feelings of resentment against another child of God, lest we wake up one morning and find ourselves stumbling around in the dark.