28 Then Manes answers him: “And what sort of God causes blindness? For it is Paul who says, In whom the God of this world has blinded the minds of them that believe not, lest the light of the Gospel should shine unto them.” But Archelaus made a good retort, saying, Read a little before: But if our Gospel is veiled, it is veiled in them that are perishing. Do you see that in them that are perishing it is veiled? For it is not right to give the things which are holy unto the dogs. Again, Is it only the God of the Old Testament that has blinded the minds of them that believe not? Hath not Jesus Himself said, For this cause speak I unto them in parables, that seeing they may not see? Was it from hating them that He wished them not to see? Or because of their unworthiness, since their eyes they had closed. For where there is wilful wickedness, there is also a withholding of grace: for to him that has shall be given; but from him that has not shall be taken even that which he seems to have.
29. “But if some are right in their interpretation, we must say as follows (for it is no unworthy expression)— If indeed He blinded the thoughts of them that believe not he blinded them for a good purpose, that they might look with new sight on what is good. For he said not, He blinded their soul, but, the thoughts of them that believe not. And the meaning is something of this kind: 'Blind the lewd thoughts of the lewd, and the man is saved: blind the grasping and rapacious thought of the robber, and the man is saved.' But will you not understand it thus? Then there is yet another interpretation. The sun also blinds those whose sight is dim: and they whose eyes are diseased are hurt by the light and blinded. Not that the sun's nature is to blind, but that the substance of the eyes is incapable of seeing. In like manner unbelievers being diseased in their heart cannot look upon the radiance of the Godhead. Nor has he said, 'He has blinded their thoughts, that they should not hear the Gospel:' but, that the light of the glory of the Gospel of our Lord Jesus Christ should not shine unto them. For to hear the Gospel is permitted to all: but the glory of the Gospel is reserved for Christ's true children only. Therefore the Lord spoke in parables to those who could not hear: but to the Disciples he explained the parables in private: for the brightness of the glory is for those who have been enlightened, the blinding for them that believe not.” These mysteries, which the Church now explains to you who art passing out of the class of Catechumens, it is not the custom to explain to heathen. For to a heathen we do not explain the mysteries concerning Father, Son, and Holy Ghost, nor before Catechumens do we speak plainly of the mysteries: but many things we often speak in a veiled way, that the believers who know may understand, and they who know not may get no hurt.
Source: Catechetical Lectures (New Advent)