If they're calling it the devil, chances are that reason wont get to them (i.e. explaining studies done on brain structure in MtF, how it is not a choice, how pretty much the whole medical establishment thinks it is real, etc.).
Do you have any family members that understand and are sympathetic to you? Grandparents, aunts, uncles, parents' cousins, etc.? Good neighbors, a family doctor, a therapist, or maybe someone from their church are possibilities too (although I know church is a long shot, assuming they go to one, but it might be worth talking to the preacher just to see). I think that if you can get someone who has connected with them and that they respect to stand up for you, that might help them in accepting you. Basically, I think you need someone who can convince them that this has nothing to do with the devil.