"And this is," it says, "the law of the ram which is for the trespass; the holy of holies. In the place in which they slaughter the whole burnt offerings, they shall also slaughter the ram which is for the trespass, before the Lord" and so forth. In the divine Scriptures it seems that frequently sin is named for trespass and trespass for sin indifferently and without any distinction; in this passage, however, it is found to be distinguished. For when above, in the victim "for sin", it had delivered the rite and order of the sacrifice, now it separately enjoins a sacrifice "for the trespass"; and although all things are enjoined in the same order and with the same observance, and it adds at the very end: "as that which is for sin, so also that which is for trespass; there shall be one law for them", yet it wished to show that there is some difference in those things for which it enjoined divided sacrifices. In which I for my part think that the trespass committed is somewhat lighter than the sin. For we find it said of sin that there is "a sin unto death", but of trespass we do not read that it is said to be unto death. I know, however, also that distinction which has seemed good to some of the wise, that a trespass is when we do not do the things we ought to do; but a sin is when we commit the things we ought not to commit. But since we do not always find these things under this distinction in the divine Scriptures, therefore we cannot affirm it as a general rule. Therefore, in the same manner and by the same tradition, the sacrifice which is offered "for the trespass" is to be understood as we above expounded concerning that which is "for sin". For similarly there are commanded to be placed upon the altar "these fats of the ram which are around the kidneys, and those which cover the inward parts"; so that you too, who hear these things, may know that everything within you that is grosser and covers your "inward parts" you ought to offer to the fire of the altar, so that all your "inward parts" may be purged, and you too may say, as David also said: "bless the Lord, O my soul, and all my inward parts his holy name". For unless that grossness which covers your "inward parts" is removed, they cannot grasp a subtle and spiritual sense, nor can they grasp the understanding of wisdom, and therefore they cannot praise the Lord. But if everything that is fat is removed from the kidneys and from all the inward parts of the entrails, then truly, purged of every vice of lust, you have slain the victim "for the trespass" and have offered a sacrifice "to God for an odor of sweetness". "The priest's," it says, "who offers it and shall make propitiation for the trespass, it shall be his". Let the priests of the Lord who preside over the churches learn that a share is given to them with those whose trespasses they shall have propitiated. But what is it to "propitiate a trespass"? If you take up a sinner and, by admonishing, exhorting, teaching, instructing, lead him to repentance, correct him from error, amend him from vices, and make him such that God becomes propitious to him upon his conversion, you will be said to have "propitiated for the trespass". If, then, you are such a priest, and such is your teaching and your discourse, a share is given to you of those whom you shall have corrected; so that their merit is your reward and their salvation your glory. Or does not the Apostle too show this, where he says: "what a man has built upon, he shall receive a reward"? Let the priests of the Lord therefore understand where a portion has been given to them, and let them be free for this and give their labor to these things. Let them not entangle themselves in vain and superfluous doings, but let them know that they will have a part with God in nothing else except in that which they offer "for sins", that is, in that they have turned sinners from the way of sin. This too is to be noted, that the things which are offered in the "whole burnt offering" are the "inward parts"; but that which is exterior is not offered to the Lord. The skin is not offered to the Lord, nor does it go into the "whole burnt offerings". Such was also that son of Judah who was called Er, which is interpreted "skin". Therefore "he was most wicked, and God slew him", because such as these are not offered to the Lord.
Source: HOMILIES ON LEVITICUS 5