Many people believe there is a God. Many also believe Jesus lived on this earth. Jesus is the Word of God .... And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. (John 1:14) .... and not all believe that Jesus lived on this earth believe in God's Word.
At one time I was engaged to marry a man named Chuck who said he was a Christian. One day he told me about seeing a movie which was about Jesus. Chuck said he just couldn't understand how the people who saw all the miracles Jesus did, didn't believe in Jesus. Surely, Chuck would say he believed in God and believed in Jesus.
But when his son planned to marry a divorced woman and I told him his son and the woman he planned to marry would commit adultery if they married, he wanted nothing to do with this. I shared scripture with him which tell us this is the case, including:
Matthew 5:32 But I (Jesus) say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.
Mark 10:11 And he saith unto them, Whosoever shall put away his wife, and marry another, committeth adultery against her. 12 And if a woman shall put away her husband, and be married to another, she committeth adultery.
If he had believed in Jesus, the Word given to us by God on this matter, he would have tried to stop his son from committing adultery by marrying the divorced woman. But he didn't believe, and turned from me instead of trying to turn his son from committing adultery.
After we broke our engagement, the Holy Spirit spoke to me: Be ye not unequally yoked together with unbelievers .....
Although many consider themselves to be Christians, some of these are unbelievers, not believing in Jesus, the Word.