I quite like Tom's version.
Heresy (let's spell it right guys) was exactly the charge levelled at Galileo. He was found guilty and subsequently confessed to this crime. Its exactly the right term to use. In fact, we may be able to lift some of Galileo's retraction into part of the certificate wording.
I ... abandon the false opinion that the Sun was the centre of the universe and immoveable, and that the Earth was not the centre of the same and that it moved, and that I was neither to hold, defend, nor teach in any manner whatever, either orally or in writing, the said false doctrine. I have been judged vehemently suspected of heresy, that is, of having held and believed that the Sun is the centre of the universe and immoveable, and that the Earth is not the centre of the same, and that it does move.
I abjure with sincere heart and unfeigned faith, I curse and detest the said errors and heresies, and generally all and every error. And I swear that for the future I will neither say nor assert in speaking or writing such things as may bring upon me similar suspicion; and if I know any heretic, or one suspected of heresy, I will denounce him.
I Galileo Galilei aforesaid have abjured, sworn, and promised, and hold myself bound as above; and in token of the truth, with my own hand have subscribed the present schedule of my abjuration, and have recited it word by word. In Rome, at the Convent della Minerva, this 22nd day of June,
So the wording is kind of olde wurlde, but it says the things we should be asking. That you renounce your globular assertions, will cease to teach these false opinions and that you pledge to uphold the beliefs of the society. The society is built on tradition, we inherited it. The wording should hark to the core histories of Flat Earth Theory.
Boom. Job done. Lets face it, for a lot of people, its meant to be a bit of fun, and its the kind of thing they expect from a bunch of nut-jobs anyway.
I quite like Tom's first draft. The wording could be tweaked, but the sentiment is about right.