If you're interested in going to a rave, maybe take them to one, or any electronic music concert just to sort of introduce them to the environment and people who will be rolling. Just an idea. Maybe if they see other people doing it safely they might open their mind up a little more. Then again, if there's anything negative at the event it may backfire on you completely.
My only other advise is to learn as much as possible about MDMA yourself. Know what it's all about, how to use it responsibly and all the negative and positive effects of it. If they ever start to change their minds and want to listen to you about it, being an expert can really help cull their fears about the substance.
Oh, and maybe try talking to them one on one. I had two friends who would be against me on a subject, but if I talked to them one on one they were more likely to listen to me and side with me. When they were together they would just gang up on me.