Then Nathanael declared, “Rabbi, you are the Son of God; you are the King of Israel.”
When Nathanael becomes convinced, he is totally, completely convinced. He is a student of the scriptures, and he immediately puts it all together and knows Jesus to be the Messiah. The Son of God and the King of Israel are both Messianic titles. Notice that Jesus had called Nathanael a true Israelite and Nathanael calls Jesus the “King of Israel.” This implies that Jesus will be Nathanael’s King from now on. It is interesting to note that the title Nathanael ascribes sincerely to Jesus at the beginning of His ministry is given to Him in mockery at the end (Matt 27:41-42).
Matt 27:41-42 In the same way the chief priests, the teachers of the law and the elders mocked him. “He saved others,” they said, “but he can’t save himself! He’s the King of Israel! Let him come down now from the cross, and we will believe in him.
There is a difference between head-belief and heart-belief. Nathanael’s enthusiasm showed his faith in Who Jesus was and indicated that he was putting his trust in Christ. Intellectual belief that Jesus is the Son of God will not save you. Even the demons believe in Who He is (Mark 3:11). A true relationship with Christ comes only from placing your trust (faith) fully in Him and in Who He is (Rom 10:10).
Mark 3:11 Whenever the evil spirits saw him, they fell down before him and cried out, “You are the Son of God.”
Romans 10:10 For it is with your heart that you believe and are justified, and it is with your mouth that you confess and are saved.