Jesus answered. He does not reprove Nathanael as if he had been too easy of belief, but rather approving of his faith, promises to him and to others that he will confirm it by stronger arguments. Besides, it was peculiar to one man that he was seen under a fig-tree by Christ, when absent and at a distance from him; but now Christ brings forward a proof which would be common to all, and thus -- as if he had broken off from what he originally intended -- instead of addressing one man, he turns to address all.

Greater things - Fuller proof of his Messiahship, particularly what is mentioned in the following verse.

Because I said - I saw thee, etc. - As thou hast credited my Divine mission on this simple proof, that I saw thee when and where no human eye, placed where mine was, could see thee, thy faith shall not rest merely upon this, for thou shalt see greater things than these - more numerous and express proofs of my eternal power and Godhead.

Jesus answered and said unto him,.... Not as reproving him for his faith, as if he was too credulous, and too easily gave into the belief of Christ, as the Son of God, and true Messiah, upon these single expressions of his; but as approving of it, and surprised at it:
because I said unto thee, I saw thee under the fig tree, believest thou? some read the words, not by way of interrogation, but as an assertion; "thou believest", or "hast believed", as the Vulgate Latin, Arabic, and Persic versions:
thou shall see greater things than these; meaning that he should have larger discoveries of his person, nature, and perfections, and should see things done by him, much more surprising than any thing he had seen yet; referring to the miracles of raising the dead, casting out devils, and healing all manner of diseases.

Because I said, &c.--"So quickly convinced, and on this evidence only?"--an expression of admiration.