One reason may be because they’ve never really understood what Jesus Christ has done for them, and what they should do in response. In other words, they only believe in Jesus in a shallow or superficial sense; they’ve never fully understood who He is or what He requires of them.
Who was Jesus? Jesus (the Bible tells us) was the Son of God, who came down from Heaven to save us from our sins. Sin isn’t just a bad habit or a flaw in our character (although it is these). Sin is rebellion against God, and as such it cuts us off from God. But Jesus Christ came to bridge that gap—and He did it by giving His life for us. He was without sin, but on the cross all our sins were placed on Him, and He took upon Himself our guilt and our shame. God loves us that much!
What, then, does it mean to believe in Jesus? It means first of all to believe that He was who He said He was: the divine Son of God, sent from Heaven for our salvation. But the word “believe” has a much deeper meaning. It also means to trust—and when we truly believe in Christ, it means we trust Him alone for our salvation.
Pray for those around you, and ask God to open ways for you to share Christ with them. Pray too that your life will be a witness to His love, and “that you may live a life worthy of the Lord … bearing fruit in every good work” (Colossians 1:10).