A heresy is any teaching or belief that stands against what is clearly stated in the Bible. John warned against heresies that denied the deity of Jesus or proclaimed another way to the Father aside from trusting in Jesus. Even today, we must also be on guard for subtle heresies that sneak into our thinking.
Pastor and author A.W. Tozer stated that "when large numbers of adherents in the Christian churches come to believe that God is different from who he actually is, that concept becomes heresy of the most insidious and deadly kind!" Tozer taught that such heresy begins when we emphasize one attribute of God above all the other attributes. In other words, we can be holding to truths about God, but favoring certain truths to the point where we disregard the totality of God's character.
I see a significant danger in the modern trend to exclusively emphasize God's love. The Bible has much to say about God's love. In fact, God is love. Jesus told us that all of the prophets and the law could be summed up in the commandment of love—loving God and loving others. Although there is obviously great importance placed on God's love, it does not cancel out God's holiness, justice, and righteousness.
On the basis of his holiness and righteousness, this God of love will condemn people to hell. He has set out rules for holy living that are not to be taken lightly. He calls certain actions, thoughts, and lifestyles sinful and destructive.
I teach in the arena of sexuality and marriage, and the "heresy of love" is particularly evident. Many Christians make decisions on what God allows based on the belief that a loving God would want them to be happy and fulfilled. I've met with women who use this logic to justify sex outside of the covenant of marriage, having affairs, and using mommy porn. I remember meeting with one married woman who was convinced that God brought another man into her life because he knew how lonely she was in her marriage. She and her "lover" prayed together and read the Bible, all the while justifying their affair. This woman could find verses in the Bible to remind her of God's compassion and love. But was she simply skipping the passages that demand holiness and self-denial?
God is the Father who waits for the prodigal to return. He is the Shepherd who leaves the flock to find the lost sheep. He is the merciful Savior who gave his life to cleanse our sin. He is the Man of Sorrows who is close to the brokenhearted. But let's not forget that he is also the righteous judge who will bring hidden things into the light. He is the God of wrath who destroys the wicked. He is the spirit of truth who rebukes and calls sinners to repentance. He is the master who told his dearest friends to suffer and deny themselves for his sake. He is holy. But we must be careful and guard against selectively emphasizing one attribute of God's character over another so that we don't create the latest heresy.