Question:
I remember being told that if you hate someone you were never considered a Christian, but how do you know if you really truly hate someone and where is that found?

Answer:
The Scripture you were told about may have been 1 John 3:15. “Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.” The Lord Jesus said that not only murder, but being “angry with his brother without cause” and being a character assassinator by saying: “Thou fool shall be in danger of hell fire” (Matthew 5:21, 22). Murder does not begin with the outward act but in the heart, and the old nature that we have since our birth to our natural parents is capable of doing any sin.

Let us look at 1 John 3 to see what we are being taught in that portion. Verse 9 says: “Whosoever is born of God does not commit [practice] sin.” This verse says that one who is born of God does not “practice,” or is not characterized by sinning. It is not a normal thing for him to sin. The next verse says that the children of God and the children of the devil are “manifest” by their practice or actions. We also learn from verse 14 that our actions show to us (ourselves) if we are saved or not. “We know that we have passed from death unto life, because we love the brethren.” But of the unsaved he says: “He that loveth not his brother abideth in death.”
The same thing (the old nature or the flesh) that produces the sin of murder leads us to hate our brother. A professed Christian woman once said between her clenched teeth, “I wish she were dead.” Yes, a Christian woman said that. But that was just for a moment, when she allowed the old nature to assert itself. She soon judged it and put it away. If Christians get out of touch with God there will be a manifestation of the flesh. But they are indwelt by the Holy Spirit and He will soon make them intensely miserable, and cause them to judge the flesh and put it in the place of death. “Grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption” (Ephesians 4:30). But no one who willfully goes on hating his brother has any business calling himself a child of God, because the above Scriptures say that no one who is characterized by hatred has eternal life abiding in Him.We should not be occupied with how much hatred we have, but with the Lord Jesus Christ as we see Him in the Word, so the Spirit of God can “transform” us into the image of Christ (2 Corinthians 3:18).