For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Only God knows who his true believers are, because only God can look into our hearts. As the apostle Paul said "if you confess with your mouth and believe in your heart, you will be saved" (Romans 10:9). Consider what God said to the prophet Samuel "But the Lord said to Samuel, “Do not consider his appearance or his height, for I have rejected him. The Lord does not look at the things people look at. People look at the outward appearance, but the Lord looks at the heart.” (1 Samuel 16:7).
God often chastised the Jewish religious leaders, because they put on a big show of the religious fervour with the sacrifices and outward appearances, but their hearts were far from God. Jesus scolded the teachers of the law and Pharisees as hypocrites and blind guides. "Woe to you, teachers of the law and Pharisees, you hypocrites! You are like whitewashed tombs, which look beautiful on the outside but on the inside are full of the bones of the dead and everything unclean" (Matthew 23:27).
But Jesus also told us we could know who the true believers were by the fruit of their lives. A good tree bears good fruit (Matthew 7:16-18). A good person, who has been born again, produces good deeds that come from a good heart.
What is the difference between a "cultural Christian" who wants religion in their life and a "genuine/authentic/true believer" who wants to glorify God with everything they think, say, and do? What does that difference look like?