Modern translations of today's Gospel (click here for Jn. 1.43-51) translate Our Lord's greeting to Nathanael, later one of His Twelve Apostles, as, "...an Israelite in whom there is no deceit!" Personally, while this is a more approachable rendition, doesn't have the same visceral punch as the old translation of, "...an Israelite in whom there is no guile!"
Guile is related to our word gullible. For someone to be victimized by their gullibility, they must fall prey to someone exercising guile, that is the intentional misleading of someone to believe a bald faced lie for all sorts of motives (my brother once, in a very guileful moment, convinced the girlfriend of our cousin that the word gullible meant fat, all for the sake of his amusement and the confusion of others, especially since the poor child indeed was quite gullible in the real sense). Someone who has guile is untrustworthy, whose word (and thoughts) cannot be trusted, whereas Nathanael had none of that, and by extension was a man of deliberation, seeking truth and speaking truth. He was not easily swayed, and he could be trusted never to exercise any guile in what he reported.
This was a very important quality Our Lord immediately fastened upon. Not only was He sent to proclaim the Gospel to the needy and to the lost, but he needed trustworthy people, people without guile, to proclaim the same. Nathanael, not one to fall onto a bandwagon because someone said so, had to see for himself, and what he saw he in his guilelessness proclaimed abroad. May we too, when confronted with the Lord, equally without guile proclaim Him.