(1) This is very blunt about final judgment with eternal consequences commonly (though somewhat imprecisely) known as "hell." Multiple speakers in the Bible, especially Jesus, are blunt about this. In my preaching I do not talk about this often enough not-in-code.
Some say that presenting the gospel as "turn or burn" is insensitive to and therefore ineffective in our culture. They make a good point. But on the other hand, our culture's abhorrence of the idea is exactly the reason people need to hear it. If it is true, then it is highly important, and people are unlikely to stumble onto this truth by accident.
(2) Yesterday I walked through part of my town praying for the people who live in the houses and apartments I was passing and for their status as the last judgment. I'm sure most of them believe in God. They don't know that the devil does too but it's not helping him much.
Paul says that people "who do not know God" are in danger. That might be a useful element in a conversation about spiritual things. "I believe in the President of the United States in the sense that I believe that there is such a person. But I don't know him. Even if I studied and learned a ton about him, I still wouldn't know him. Do you think it's possible to actually know God?"
(3) In those homes I passed, I also suspect that most of them believe "the gospel of our Lord Jesus." In my town there's better than 50% odds that if I asked a random person, "Do you believe that Jesus is the Son of God, that he died and rose again, that your badness can be forgiven and that you can have eternal life because of him?", that person would agree.
But not nearly as many people in those homes obey the gospel. They do not live differently because they believe those principles than if they did not. This is another valuable element in a spiritual conversation: "If we don't obey the message about Jesus, we are still in danger."