The key, at least for me, is to build up your own self-confidence and objectivity. You know it's a cult. You know to participate in its activities makes you complicit in fraud, abuse and even (via the blood policy and HLC manipulation) murder. Why would you ever feel guilty for failure to participate? You ought to be proud that you've taken a stand against the evils of the cult at great cost to you.
What worked for me while I had lingering feelings of guilt/doubt was to mentally walk myself down the path that I'd taken that led me to leaving. Walk back past all the deaths the cult has caused (blood ban, malawi, organ transplant ban, etc etc) and all the money it's taken and all the child victims of pedophiles it's turned it's back on and all the lies it's told you. If you do this enough times when you start to feel doubt/guilt, it becomes second nature and you know the path so well that you skip over the guilt altogether.
Now that's probably not an effective response to your parents, but it's the first part of having one - it will help you to be unaffected by their appeals which will be instrumental in stopping them doing it. My response to attempts to make me feel guilty by pointing out the fact of my lack of participation in cult activities is simply to acknowledge the facts, then offer to give them my reasons if they'd like to hear it. Getting emotional (any emotion, as it turns out) will only validate their preconceived notions. If you respond with guilt, they'll see it as "you know it's the truth." Anger makes you a bitter apostate. Exasperation at their ignorance means you're condescending and puffed up with pride. In my experience, dispassionate fact-based discourse quickly shuts down the conversation because it dispels any notion that they might have of manipulating you, and they certainly can't suffer facts long as they quickly being to cast the cult in a bad light. They know this on some level and therefore avoid the conversation altogether.