17 What do we then, brethren? Ought this man to be the first among the apostles? Not only is Nathanael not found as first among the apostles, but he is neither the middle nor the last among the twelve, although the Son of God bore such testimony to him, saying, “Behold an Israelite indeed, in whom is no guile.” Is the reason asked for? In so far as the Lord intimates, we find a probable reason. For we ought to understand that Nathanael was learned and skilled in the law and for that reason was the Lord unwilling to place him among His disciples, because He chose unlearned persons, that He might by them confound the world.
Listen to the apostle speaking these things: “For you see,” says he, “your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called: but God has chosen the weak things of the world to confound the things which are mighty; and base things of the world, and things that are despised, has God chosen, yea, and things which are not, as though they were things that are, to bring to nought things that are.” If a learned man had been chosen, perhaps he would have said that he was chosen for the reason that his learning made him worthy of choice.
Our Lord Jesus Christ, wishing to break the necks of the proud, did not seek the orator by means of the fisherman, but by the fisherman He gained the emperor. Great was Cyprian as an orator, but before him was Peter the fisherman, by means of whom not only the orator, but also the emperor, should believe. No noble was chosen in the first place, no learned man, because God chose the weak things of the world that He might confound the strong. This man, then, was great and without guile, and for this reason only was not chosen, lest the Lord should seem to any to have chosen the learned.
And from this same learning in the law, it came that when he heard “from Nazareth,”— for he had searched the Scripture, and knew that the Saviour was to be expected thence, what the other scribes and Pharisees had difficulty in knowing—this man, then, very learned in the law, when he heard Philip saying, “We have found Him, of whom Moses in the law, and the prophets, did write, Jesus of Nazareth, the son of Joseph;”— this man, who knew the Scriptures excellently well, when he heard the name “Nazareth,” was filled with hope, and said, “From Nazareth something good can come.”
Source: Tractates on the Gospel of John (New Advent)