Heart-rendering, and eerily familiar! I had a very similar conversation with my parents, too. But I got them from eventually shunning me (altho' there is a good reduction in our interactions).
Here was the part that struck a cord in them:
Says I: "Mom, Dad you do this (shunning) for God right? Well, you think that makes it right? Now the Boko Haram (a terrorist Islamist group in my country) carry out suicide bombings everyday, and they claim they do it for God, for what they believe in. Do you think they were sincere in practising their beliefs? You feel they are sincere, right? Now does that ever justify what they did? Nope! Yes, doing something bad in God's name would never make it right! And you know shunning someone because of ideological differences is very bad- you guys taught us how bad racism and tribalism is, now this is worse than them both! And Dad, your greatgrandfather was an Ifa priest, his son, your grandfather was an Oshin worshipper and your father converted to what you now call part of Christendom. You, Dad chose to become a JW. Now, Dad, down history our family has been changing religious affliations down to you, has you heard of any disowning or shunning his progeny because of it? Now, if you belong to a religion that claims to be liberal, fair and has claims of "secrets to family happiness" tells you to shun your child, and create an emotional turmoil for all the family, something even the so-called heathen religions didn't do nor Christendom did, now tell me Dad and Mum, how accommodating and loving is your religion? So, if you allow a religion to dictate an unloving law to you, and you follow it and say you did it 'cause God said it, you are no better than Boko Haram members and I would also gladly divest myself of parents who have the mindset of terrorists!"
Now that really got them thinking, and they decided not to shun me again!