Albert Barnes on John 1:49

Rabbi. Master. Applied appropriately to Jesus, and to no one else, Mt 23:10.

The Son of God. By this title he doubtless meant that he was the Messiah. His conscience told him that he had judged right of his character, and that therefore he must know the heart and the desires of the mind. If so, he could not be a mere man, but must be the long-expected Messiah.

The King of Israel. This was one of the titles by which the Messiah was expected, and this was the title which was affixed to his cross, Jn 19:18. This case of Nathanael John adduces as another evidence that Jesus was the Christ. The great object he had in view in writing this gospel was to collect the evidence that he was the Messiah, Mt 20:31. A case, therefore, where Jesus searched the heart, and where his knowledge of the heart convinced a pious Jew that he was the Christ, is very properly adduced as important testimony.

(p) "the Son" Mt 14:33, Jn 20:28,29 (q) "the King of Israel" Mt 21:5, 27:11