Augustine of Hippo — as excerpted in the Catena Aurea on John 6:60-71

“(Tr. xxvii. s. 7) He says not, There are some among you who understand not; but gives the reason why they do not understand. The Prophet said, Except ye believe, ye shall not understanda. (Is. 7:9) For how can he who opposes be quickened? An adversary, though he avert not his face, yet closes his mind to the ray of light which should penetrate him. But let men believe, and open their eyes, and they will be enlightened.”