"Even if it isn't the truth, it's still the best way to live"
OMG! I have heard that verbatim from the mouth of an elder, no less!
But it's a lie! And you can use their own bible to show them it's a lie. You just have to tell them about that scripture in 1 Corinthians 15 where Paul says that if the dead are not to be raised up then Christians are most to be pitied. Paul's statement is an implicit admission that living as a christian is definitely miserable way of life filled with hardship and sacrifice and is only worth it if the hope they look forward to is true. So if it's not true then it's not the best way of life. Paul also said that Christians must enter into the kingdom by many tribulations. Best way of life even if it isn't the truth, my ass!
I can think of better ways of living that don't involve shunning my relatives just because they no longer share my religious beliefs.
I can think of better ways of living than refusing lifesaving blood transfusions for myself or my infant child.
I can think of better ways of living than refusing my child a higher education even if I could afford it.
I can think of better ways of living than judging and mistrusting everyone around you just because they don't share your religious beliefs.
I can think of better ways of living than not marrying the person you're truly attracted to and love, just because they're not of the same religion as you.
I can think of better ways of living than wasting most or all of my free time slaving in the hot sun for a haughty, deceitful, hypocritical, manipulative, judgmental, misogynistic, false-prophesying, religious cult.