As a former Jehovah's Witness, I can tell you that the most powerful, thought provoking question I was ever asked by a "householder" when I was engaged in the "field service" ministry was this:
"What if everything you have ever believed about God and the Bible was wrong?"
That was 25 years ago, and I can tell you--that one question stuck with me, like a "sliver" in my heart for all these years. Last year in July when I became "born-again" in Christ, I remembered the huge impact that this one simple straight-forward question had on me.
The gal who posed the question, probably never gave it another thought! Yet for me, it planted a "seed".
Not a seed of truth. Not the "seed" of salvation through Christ.
But a "seed" of doubt. She did not need to say anything else to me.
I wish I could thank her now! You never know what the "key" will be to unlock a person's heart. So just pray and trust as you open the door to these folks--that God will give you the right key!
What a wonderful question you posed!