13 “Blessed are you,” says He, “when men shall revile you, and persecute you, and shall say all manner of evil against you falsely for my sake. Rejoice and be exceeding glad: for great is your reward in heaven.” Let any one who is seeking after the delights of this world and the riches of temporal things under the Christian name, consider that our blessedness is within; as it is said of the soul of the Church by the mouth of the prophet, “All the beauty of the king's daughter is within;” for outwardly revilings, and persecutions, and disparagements are promised; and yet, from these things there is a great reward in heaven, which is felt in the heart of those who endure, those who can now say, “We glory in tribulations: knowing that tribulation works patience; and patience, experience; and experience, hope: and hope makes not ashamed; because the love of God is shed abroad in our hearts by the Holy Ghost which is given unto us.” For it is not simply the enduring of such things that is advantageous, but the bearing of such things for the name of Christ not only with tranquil mind, but even with exultation. For many heretics, deceiving souls under the Christian name, endure many such things; but they are excluded from that reward on this account, that it is not said merely, “Blessed are they which endure persecution;” but it is added, “for righteousness' sake.” Now, where there is no sound faith, there can be no righteousness, for the just [righteous] man lives by faith. Neither let schismatics promise themselves anything of that reward; for similarly, where there is no love, there cannot be righteousness, for “love works no ill to his neighbour;” and if they had it, they would not tear in pieces Christ's body, which is the Church.
14. But it may be asked, What is the difference when He says, “when men shall revile you,” and “when they shall say all manner of evil against you,” since to revile is just this, to say evil against? But it is one thing when the reviling word is hurled with contumely in presence of him who is reviled, as it was said to our Lord, “Say we not the truth that you are a Samaritan, and hast a devil?” and another thing, when our reputation is injured in our absence, as it is also written of Him, “Some said, He is a prophet; others said, Nay, but He deceives the people.” Then, further, to persecute is to inflict violence, or to assail with snares, as was done by him who betrayed Him, and by them who crucified Him. Certainly, as for the fact that this also is not put in a bare form, so that it should be said, “and shall say all manner of evil against you,” but there is added the word “falsely,” and also the expression “for my sake;” I think that the addition is made for the sake of those who wish to glory in persecutions, and in the baseness of their reputation; and to say that Christ belongs to them for this reason, that many bad things are said about them; while, on the one hand, the things said are true, when they are said respecting their error; and, on the other hand, if sometimes also some false charges are thrown out, which frequently happens from the rashness of men, yet they do not suffer such things for Christ's sake. For he is not a follower of Christ who is not called a Christian according to the true faith and the catholic discipline.
15. “Rejoice,” says He, “and be exceeding glad: for great is your reward in heaven.” I do not think that it is the higher parts of this visible world that are here called heaven. For our reward, which ought to be immoveable and eternal, is not to be placed in things fleeting and temporal. But I think the expression “in heaven” means in the spiritual firmament, where dwells everlasting righteousness: in comparison with which a wicked soul is called earth, to which it is said when it sins, “Earth you are, and unto earth you shall return.” Of this heaven the apostle says, “For our conversation is in heaven.” Hence they who rejoice in spiritual good are conscious of that reward now; but then it will be perfected in every part, when this mortal also shall have put on immortality. “For,” says He, “so persecuted they the prophets also which were before you.” In the present case He has used “persecution” in a general sense, as applying alike to abusive words and to the tearing in pieces of one's reputation; and has well encouraged them by an example, because they who speak true things are wont to suffer persecution: nevertheless did not the ancient prophets on this account, through fear of persecution, give over the preaching of the truth.
Source: Our Lord's Sermon on the Mount (New Advent)