Well, a few things I can say. Men by their nature will find anything to justify their sin so they can live in it. Whether it be, "I don't believe in God because I lived with Christian parents that were very mean." or anything else. There's always an idol in the way of that person, and it's something that they want from it, that they would say that God is not real. We can put many excuses out that "Oh because of that, I don't believe in God." when the root issue here is just pride.
One thing we can learn from this Christian, is that the way you live out for Christ shows the Gentiles (people who don't believe in God), what being a Christian really means. It's not some religion where you perform rules and rituals, but it's being reconciled from sin, and being closer to God.
Watch 16:18-19:41Your Life, Witness, and Testimony as a Christian - Tim ConwayRomans 1:19-21
For what can be known about God is plain to them, because God has shown it to them. 20 For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world,[g] in the things that have been made. So they are without excuse. 21 For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened.
Psalms 14:1
The fool says in his heart, “There is no God.” They are corrupt, they do abominable deeds, there is none who does good.
If you and I went to Paris and we looked at the Eiffel tower, and then you said to me, "Wonder how that came to be, someone didn't design it though." I would think you're crazy! By the sheer mass and design of that building I already knew a group of people made that. How much more can we say to a person who says there is no God? The vast universe, and just as something small as DNA, just proves there is a God. My biology textbook says that DNA has enough information for 1,000 textbooks. Where did that come from? Where did it all began? And as we go back to Romans 1, men have no excuse.