How are they to believe in him of whom they have never heard? Romans 10:14
Some do not believe because they have not heard. This is the reason we are to tell the good news of Jesus to every person on the planet.
This is the Great Commission. Go and tell every person, “There is good news for you! God sent his Son into the world that whoever believes in him should not perish but have eternal life.”
But many who have heard do not believe. Christ “came to his own, and his own people did not receive him” (John 1:11). The Gospels make it clear that many of the people among whom Jesus ministered did not believe in him. He was rejected by the Scribes and Pharisees, and after he performed a miracle in their town, the people of Decapolis asked him to leave their area.
Then there were people who professed to believe in him for a time, but then turned back. They were part of the group that said, “Yes, yes, we believe in Jesus,” but it did not last. You have this at the end of John 2, where we read that: “Many believed in his name when they saw the signs that he was doing. But Jesus on his part did not entrust himself to them, because he knew all people… he himself knew what was in man” (John 2:23-24, 25).
You see it again in John 6:66. The people hear teaching from Jesus that they find hard to take: “After this many of his disciples turned back and no longer walked with him.”
Do you know someone who has heard the gospel but doesn’t believe? Do you know anyone who once professed Christ but has turned back from following him?