This verse is an interesting commentary on mankind regarding the sin of lying, which is so common that it seems to be "in the genes"! Jesus seems pleasantly surprised at Nathanael. He describes a person without deceit, a simple, innocent person without subtlety, candid, and sincere. Is this a compliment or mild sarcasm? Or is He saying, "Here is a genuine Israelite, one in whom is no falsehood?" If so, He means, "This is how an Israelite should be!"
Regardless, Nathanael's lack of guile impressed Jesus, indicating its rarity. Lying is such an integral part of our lives that we have coined such expressions of disbelief as "Is that so?" "Do you really mean it?" or "You don't say!" because so many tales we hear stretch credulity.
John W. Ritenbaugh
The Ninth Commandment (1997)