Heresy charges are often added to the original charges. There will be two main defenses that might work, assuming the definition is something like that of Aquinas.
Accordingly, certain doctors seem to have differed either in matters the holding of which in this or that way is of no consequence, so far as faith is concerned, or even in matters of faith, which were not as yet defined by the Church; although if anyone were obstinately to deny them after they had been defined by the authority of the universal Church, he would be deemed a heretic.1) The teaching isn't yet a declared heresy. This requires that the following four steps be followed:
- Doctrine challenges an established teaching with a controversial teaching
- Doctrine is circulated among the believers
- Church leadership meets to deal with the new teaching
- Leadership corporately rejects the new teaching
2) If those four steps have been followed, then your defense needs to be that, as a believer, you haven't yet engaged in pertinacity. That is, you have not yet been provided with the church's counterargument--the reason the heresy was formally rejected. You cannot be guilty of heresy in ignorance.
Do not attempt or agree to argue the correctness of your belief as part of your trial. You are trying to win on a technicality. If you argue the belief, you will be excommunicated and the trial is "just for the record" (see minister trials if you want to go down that road). The problem there is that agreeing with you is essentially a declaration that the belief is not heretical. To win, you need only prove that it's not heretical for you yet. Don't let them raise the bar.
Make it clear that you agree that once the belief were declared a heresy, or once you were made aware of why it's a heresy, you would be obligated to immediately examine your conscience and either (a) come into compliance, or (b) if because of "intellectual delinquencies in myself that were involuntary and imputable", you were unable to come into compliance, you would need to ask for a letter of transfer. That makes it absolutely clear that you are agreeing to be bound by leadership without actually agreeing to agree with them in advance. If they don't find that sufficient, they are the ones on shaky ground.
For example Helena is sexual experimenting and is trying lesbian sex. She tells her best friend, Marissa that the bible doesn't condemn consensual homosexuality only prostitution. Then her friend feels comfortable fooling around. The two girls get caught and Marissa confesses everything but blames Helena. Everyone wants to excommunicate Helena and so Helena gets charged with heresy in addition to sexual immorality. She defends herself against the heresy by arguing that it was religious books make this case that "qadesh" is a temple prostitute and "to’ebah" is a specific practice. She has never claimed to be hebrew scholar herself and the church has never provided her with hebrew training. Nor had she received instruction in proper translation. Nor had she been told that the NIV's translation was authorative. It had never been explained to her why the NIV translators used "abomination". At the time she told Marissa her opinion it therefore could not have been heresy (even though it might very well have been incorrect).
The one thing Helena must do is make this case about her state of knowledge at the time she made those statements, she cannot attempt to defend that they are true or its over. She should refuse to address those issues and ask for further instruction on them.