7 “But,” says He, “there are some among you that believe not.” He said not, There are some among you that understand not; but He told the cause why they understand not. “There are some among you that believe not,” and therefore they understand not, because they believe not. For the prophet has said, “If you believe not, you shall not understand.” We are united by faith, quickened by understanding. Let us first adhere to Him through faith, that there may be that which may be quickened by understanding.
For he who adheres not resists; he that resists believes not. And how can he that resists be quickened? He is an adversary to the ray of light by which he should be penetrated: he turns not away his eye, but shuts his mind. “There are,” then, “some who believe not.” Let them believe and open, let them open and be illumined. “For Jesus knew from the beginning who they were that believed, and who should betray Him.” For Judas also was there. Some indeed, were offended; but he remained to watch his opportunity, not to understand.
And because he remained for that purpose, the Lord kept not silence concerning him. He described him not by name, but neither was He silent about him; that all might fear though only one should perish. But after He spoke, and distinguished those that believe from those that believe not, He clearly showed the cause why they believed not. “Therefore I said unto you,” says He, “that no man can come unto me except it were given to him of my Father.” Hence to believe is also given to us; for certainly to believe is something. And if it is something great, rejoice that you have believed, yet be not lifted up; for “What do you have that you did not receive?”
Source: Tractates on the Gospel of John (New Advent)