82 But seeing that, however pure an eye one may have, i.e. with however single and sincere a heart one may live, he yet cannot look into the heart of another: whatever things could not have become apparent in deeds or words, are disclosed by trials. Now trial is twofold; either in the hope of obtaining some temporal advantage, or in the terror of losing it. And especially must we be on our guard, lest, when striving after wisdom, which can be found in Christ alone, in whom are hid all the treasures of wisdom and knowledge; — we must be on our guard, I say, lest, under the very name of Christ, we be deceived by heretics, or by any parties whatever defective in intelligence, and lovers of this world. For on this account He adds a warning, saying, “Not every one that says unto Me, Lord, Lord, shall enter into the kingdom of heaven; but he that does the will of My Father which is in heaven, he shall enter into the kingdom of heaven:” lest we should think that the mere fact of one saying to our Lord, “Lord, Lord,” belongs to those fruits; and from that he should seem to us to be a good tree. But those are the fruits, to do the will of the Father who is in heaven, in the doing of which He has condescended to exhibit Himself as an example.
83. But the question may fairly be started, how with this sentence the statement of the apostle is to be reconciled, where he says, “No man speaking by the Spirit of God calls Jesus accursed; and no man can say that Jesus is the Lord, but by the Holy Ghost:” for neither can we say that any who have the Holy Spirit will not enter into the kingdom of heaven, if they persevere onwards to the end; nor can we affirm that those who say, “Lord, Lord,” and yet do not enter into the kingdom of heaven, have the Holy Spirit. How then does no one say “that Jesus is the Lord, but by the Holy Ghost,” unless it is because the apostle has used the word “say” here in a strict and proper sense, so that it implies the will and understanding of him who says? But the Lord has used the word which He employs in a general sense: “Not every one that says unto Me, Lord, Lord, shall enter into the kingdom of heaven.” For he also who neither wishes nor understands what he says, seems to say it; but he properly says it, who gives expression to his will and mind by the sound of his voice: just as, a little before, what is called “joy” among the fruits of the Spirit is called so in a strict and proper sense, not in the way in which the same apostle elsewhere uses the expression, “Rejoices not in iniquity:” as if any one could rejoice in iniquity: for that transport of a mind making confused and boisterous demonstrations of joy is not joy; for this latter is possessed by the good alone. Hence those also seem to say it, who neither perceive with the understanding nor engage with the deliberate consent of the will in this which they utter, but utter it with the voice merely; and after this manner the Lord says, “Not every one that says unto Me, Lord, Lord, shall enter into the kingdom of heaven.” But truly and properly those parties say it whose utterance in speech really represents their will and intention; and it is in accordance with this signification that the apostle has said, “No one can say that Jesus is the Lord, but by the Holy Ghost.”
84. And besides, it belongs especially to the matter in hand, that, in striving after the contemplation of the truth, we should not only not be deceived by the name of Christ, by means of those who have the name and have not the deeds; but also not by certain deeds and miracles, for when the Lord performed of the same kind for the sake of unbelievers, He has warned us not to be deceived by such things, thinking that an invisible wisdom is present where we see a visible miracle. Hence He annexes the statement: “Many will say to Me on that day, Lord, Lord, have we not prophesied in Your name, and in Your name have cast out devils, and in Your name done many wonderful works? And then will I say unto them, I never knew you: depart from Me, you that work iniquity.” He will not, therefore, recognise any but the man that works righteousness. For He forbade also His own disciples themselves to rejoice in such things, viz. that the spirits were subject unto them: “But rejoice,” says He, “because your names are written in heaven;” I suppose, in that city of Jerusalem which is in heaven, in which only the righteous and holy shall reign. “Do you not know,” says the apostle, “that the unrighteous shall not inherit the kingdom of God?”
85. But perhaps some one may say that the unrighteous cannot perform those visible miracles, and may believe rather that those parties are telling a lie, who will be found saying, “We have prophesied in Your name, and have cast out devils in Your name, and have done many wonderful works.” Let him therefore read what great things the magi of the Egyptians did who resisted Moses, the servant of God; or if he will not read this, because they did not do them in the name of Christ, let him read what the Lord Himself says of the false prophets, speaking thus: “Then, if any man shall say unto you, Lo, here is Christ, or there; believe it not. For there shall arise false Christs, and false prophets, and shall show great signs and wonders, insomuch that the very elect shall be deceived. Behold, I have told you before.”
86. How much need, therefore, is there of the pure and single eye, in order that the way of wisdom may be found, against which there is the clamour of so great deceptions and errors on the part of wicked and perverse men, to escape from all of which is indeed to arrive at the most certain peace, and the immoveable stability of wisdom! For it is greatly to be feared, lest, by eagerness in quarrelling and controversy, one should not see what can be seen by few, that small is the disturbance of gainsayers, unless one also disturbs himself. And in this direction, too, runs that statement of the apostle: “And the servant of the Lord must not strive; but be gentle unto all men, apt to teach, patient, in meekness instructing those that think differently; if God perhaps will give them repentance to the acknowledging of the truth.” “Blessed,” therefore, “are the peacemakers: for they shall be called the children of God.”
Source: Our Lord's Sermon on the Mount (New Advent)