It’s not so difficult to imagine people having different beliefs/lack of beliefs as us. Look around you and see how different each and every person looks and acts. I am not an atheist but I think a lot of people turn to atheism because we cannot see God.
Human beings are very visual in nature, and so we often cannot understand things that extend pas the scope of our vision. If you don’t already have a strong conviction in and love for the religion you were born in, atheism isn’t that far fetched. Just pray that they are guided and leave them be, no need for all this vitriol. And you can absolutely have morals without being religious.