Romans 10:9, which says, “If you confess with your mouth, ‘Jesus is Lord,’ and believe in your heart that God raised him from the dead, you will be saved.” Not, “You might be saved,” or “You could be saved.” But, “You will be saved.”
What does it mean to believe in your heart? Well, it’s more than just an intellectual thing. It’s more than just saying Jesus was a great teacher or even saying that Jesus was God’s Son. To believe means to trust him with your life, to give him your heart and soul, to be willing to follow him for the rest of your life.
The verse also says we should “confess” that Jesus is Lord. That simply means telling others that you follow Jesus. Now, telling people about Jesus doesn’t make you a Christian, but it is evidence that you are a Christian.