Matthew 5:4

Those that mourn, that is, not loss of kindred, affronts, or losses, but who weep for past sins.

(ubi sup.) When you have done thus much, attained both poverty and meekness, remember that you are a sinner, mourn your sins, as He proceeds, Blessed are they that mourn. And it is suitable that the third blessing should be of those that mourn for sin, for it is the Trinity that forgives sin.

Now having begun, as you see, where most need was, He proceeds to another commandment, one which seems to be opposed to the judgment of the whole world. For whereas all think that they who rejoice are enviable, those in dejection, poverty, and mourning, wretched, He calls these blessed rather than those; saying thus, Blessed are they that mourn. Yet surely all men call them miserable. For therefore He wrought the miracles beforehand, that in such enactments as these He might be entitled to credit. And here too again he designated not simply all that mourn, but all that do so for sins: since surely that other kind of mourning is forbidden, and that earnestly, which relates to anything of this life. This Paul also clearly declared, when he said, The sorrow of the world worketh death, but godly sorrow worketh repentance unto salvation, not to be repented of. These then He too Himself calls blessed, whose sorrow is of that kind; yet not simply them that sorrow did He designate, but them that sorrow intensely. Therefore He did not say, they that sorrow, but they that mourn. For this commandment again is fitted to teach us entire self-control. For if those who grieve for children, or wife, or any other relation gone from them, have no fondness for gain or pleasure during that period of their sorrow; if they aim not at glory, are not provoked by insults, nor led captive by envy, nor beset by any other passion, their grief alone wholly possessing them; much more will they who mourn for their own sins, as they ought to mourn, show forth a self-denial greater than this. Next, what is the reward for these? For they shall be comforted, saith He. Where shall they be comforted! tell me. Both here and there. For since the thing enjoined was exceeding burthensome and galling, He promised to give that, which most of all made it light. Wherefore, if thou wilt be comforted, mourn: and think not this a dark saying. For when God doth comfort, though sorrows come upon thee by thousands like snow-flakes, thou wilt be above them all. Since in truth, as the returns which God gives are always far greater than our labors; so He hath wrought in this case, declaring them that mourn to be blessed, not after the value of what they do, but after His own love towards man. For they that mourn, mourn for misdoings, and to such it is enough to enjoy forgiveness, and obtain wherewith to answer for themselves. But forasmuch as He is full of love towards man, He doth not limit His recompense either to the removal of our punishments, or to the deliverance from our sins, but He makes them even blessed, and imparts to them abundant consolation. But He bids us mourn, not only for our own, but also for other men's misdoings. And of this temper were the souls of the saints: such was that of Moses, of Paul, of David; yea, all these many times mourned for evils not their own.

For the mourning here meant is not for the dead by common course of nature, but for the dead in sins, and vices. Thus Samuel mourned for Saul, thus the Apostle Paul mourned for those who had not performed penance after uncleanness.

(Serm. in Mont. i. 2.) Otherwise; mourning is sorrow for the loss of what is dear; but those that are turned to God lose the things that they held dear in this world; and as they have now no longer any joy in such things as before they had joy in, their sorrow may not be healed till there is formed within them a love of eternal things. They shall then be comforted by the Holy Spirit, who is therefore chiefly called, The Paraclete, that is, 'Comforter;' so that for the loss of their temporal joys, they shall gain eternal joys.