Believe.

In John 6:29 it says, "²⁹ Jesus told them, “This is the only work God wants from you: Believe in the one he has sent.”

We have to believe in what God has said to us. He sent his one and only Son down to save us from our sins. (John 3:16)
Jesus died on the cross to save our sins. His blood was shed for us. His blood represents the blood of a lamb. Before Jesus was born, people had to sacrifice a clean and pure lamb in order for God to forgive them. This is where the name "Lamb of God" comes from.

After Jesus died, he rose 3 days later. We have to believe that he has risen and he did it to save us from our sins. Yes, we all make mistakes, we get tempted by Satan, but in the end we need to repent with the right mind and heart and promise God that we wouldn't do the sin again.

We must believe in Him in order to spend eternity with him in Heaven.