Sungenis does not have it quite right, but he has a good point. Our current apostasy can be traced back to the general acceptance of the Copernican Principle as a philosophical system. Rather than retain the philosophical viewpoint of the Bible, that the earth is the reference point for reality, church people began to judge the earth from an arbitrary and imaginary point in space. The King James Bible was no longer the standard of truth, and it was replaced by pseudoscience.
Flat earth falls short on many accounts, but the one I want to address here is its mistaking the physical as final reality. This is an error that it shares with more popular concepts, such as heliocentrism (earth going around the sun) and even geocentrism. If one accepts a physical reality as final, he cuts himself off from being able to see that his model is wrong. Models proposed based on physical observation do not establish final reality.
Hebrews 11:3 (KJV)
Through faith we understand that the worlds were framed by the word of God, so that things which are seen were not made of things which do appear.