The thing that brands many people as heretics in the eyes of the Christian fundamentalists is the belief that God is 100% successful in all His efforts.
Fundamentalists instead believed that God allows man's free will to thwart His divine will. They also accept that Satan, a supposedly created being, exercises supernatural influence over mankind, further subverting God's creative purpose. It should be obvious that such beliefs diminish God and reduce Him to a subordinate role within His own creation. If Satan's intervention and man's exercise of free will were not a part of God's plan, leading ultimately to the complete accomplishment of all that He intended in the first place, God becomes a failure, a most ungodly quality.
The only way around this conclusion is to assume that God's ultimate purpose for mankind was to condemn the vast majority in some sort of cosmic culling exercise. Such an assumption flies in the face of the Bible and again paints a most unflattering picture of God.
Thus, some are branded as heretics for believing in a God who is too big, too marvelous, too compassionate, too radical for the carnal mind to accept. If Jesus was not the very image of this grander God, then what was He? If Jesus just came to reinforce man's longstanding picture of lesser gods, then why was he dismissed and ultimately killed as a dangerous radical, religiously and politically? It is hard to image that the human mind could invent a Being more majestic and transcendent than God actually is. Is that what we are actually willing to believe and accept?