What heresy? Mormonism is viewed as the heresy by mainstream Christians. We also view Joseph Smith as a false prophet who had many prophecies that did not come to pass. Deuteronomy 18:22 goes to the heart of this issue and flatly states that if any man who issues a prophecy in the name of the Lord and that prophecy does not come to pass that man is false. Joseph Smith failed the test of Deut. 18:22 not once but successive times, which proves that any other teaching done by him or any of his successors being equally false. I believe following a false prophet would be the real heresy here.
What you are going to have to do is support your position that Smith was a true prophet of God having violated your own LDS teaching regarding Deut. 18:22.