You're viewing a single thread.
I typically use the fact that there are trees older than 4000 years old based on tree ring data. Or that there are stars in the sky further than 4000 light years away that we can see in the sky.
That usually makes them say something like how their God created an world that was already aged. So I usually counter with the fact that would make their God a lier and deceiver.
Some hold firm and say God did it to test faith. Others back pedal and try to blame it on Satan. That Satan scattered all this false evidence just to make us question the notion that Earth is 4000 years old to make people lose faith in God. And then I have to laugh at how stupid their argument is and how weak their God is. Naturally no amount of evidence or logic will make them change their belief.
The important thing is, you're compelling people to examine their pre-existing beliefs. They won't change their beliefs during your conversation, because deprogramming takes time. But the more seeds of doubt you plant, the better the chances are that some will germinate.
I find that the most effective way to encourage people to question themselves is to discuss things calmly and in good faith, through in-person conversations. Challenging people to "convert me" has been surprisingly fruitful - after all, I honestly would love to believe that a benevolent deity is looking out for us all. (As well, tons of believers would equally love to be the one who "shows [you or me] the light.") I want them to provide compelling evidence that can change my mind.
Approaching the conversation in this fashion not only challenges the "missionary" types to think harder, but it also shifts the onus onto them to convince you. If they've never thought critically about their message, this kind of conversation may introduce questions that stick with them long after it's over.