Christians become Christians by searching and considering if Jesus, God and the Bible is indeed true. They find a point of truth strong enough to dedicate their lives to it. Even to die for it. It means living honorably and being blameless for causing harm. Naturally they won't like the basis for their entire life's meaning to be ripped out from under them.
Put yourself in the role of a serious believer. IF God and the Bible are real... there's eternal life in heaven to be had. There's life improvement through prayer. There's 7000 promises of blessing. There's strength in God for all we can't do on our own. There's promise of protection. There's the promise to do whatever we ask, as long as it's beneficial to God. He promises all pain to be there to grow us and bring us closer to Him, and to use it for good, so it's never meaningless. He promises that our plans will succeed, if they're committed to His purposes.
That's a lot of treasure and meaning for living an honorable life.... doing all the things the world needs most and committing to not being part of anything more destructive.
To say it's all a lie means not just taking away the greatest hope for humanity and the greatest force for all things good and pure and beautiful... but it encourages people to ignore it and miss it.
Living apart from God's will may seem like freedom, but it's false freedom. We're free to run wild and get caught up in everything He wants to protect us from. It never goes anywhere good for long.
The worst part of it? Many people use "God doesn't exist" as justification to do horrible things. Selling drugs, sex trafficking, stealing, cheating, even murder. If someone feels there's no punishment given they don't get caught... what's stopping them?
Not all unbelievers commit crimes. But all who commit crimes believe there's no real punishment.
We cannot encourage disbelief in God without encouraging the very kinds of destruction that can destroy us all.