Thomas Aquinas Commentary 1 Corinthians 5:6-8

"Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? Purge out the old leaven, that ye may be a new lump, even as ye are unleavened. For our passover also hath been sacrificed, [even] Christ: wherefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth." — 1 Corinthians 5:6-8 (ASV)

After reminding the Corinthians of two crimes—namely, the fornicator's and that of those who condoned his sin—the Apostle now censures both. He first addresses the crime of condoning the sin, and second, the sin of the fornication itself (in chapter 6). Concerning the first crime, he does two things: first, he rebukes the Corinthians for failing to pass judgment; second, he addresses other vices related to judgment (in chapter 6). Regarding their failure to pass judgment, he again does two things: first, he blames them for not casting out the fornicator; second, he corrects the false understanding they took from his words (in verse 9).

Regarding the first point, he does two things: first, he rebukes what they had done, and second, he shows what should be done (in verse 7). He begins by rebuking their past crime at its root, for he had said above that because they were "puffed up," they lacked compassion, which led to their failure to correct others and set them straight. Therefore, he first censures them for being puffed up, saying, Your boasting is not good, because you boast in the defects of others as though you were without faults yourselves.

Everyone should boast in the blessings given to them by God and not in others: Let each one test his own work, and then his reason to boast will be in himself alone and not in his neighbor (Galatians 6:4). And it is especially evil to boast in the failures of others: Why do you boast of mischief? (Psalms 51:3).

Second, he gives the reason for his rebuke, asking, Do you not know that a little leaven leavens the whole lump? As if to say, "Surely you cannot be unaware of this." It should be noted that there are two aspects to consider in leaven. The first is the flavor it gives to bread. In this sense, leaven signifies the wisdom of God, through which everything human is made savory. Accordingly, it says in Matthew 13:33, The kingdom of heaven is like leaven which a woman took and hid in three measures of meal, till it was all leavened.

The second aspect of leaven is corruption. In this sense, leaven can signify sin, because by one sin all of a person's works are corrupted. For example, the sin of hypocrisy is compared to leaven in Luke 12:1: Beware of the leaven of the Pharisees, which is hypocrisy. In another way, a sinful man himself can be signified by leaven.

This is precisely the point of his metaphor. Just as the entire lump of dough is corrupted by a little leaven, so a whole group can be defiled by one sinner: From one spark comes a great fire and from one deceitful man much blood . This happens when the sin of one person leads others to sin, or even when they consent to his sin by failing to correct him when they are able: They are worthy of death, not only who do these things but also who consent to those who do them (Romans 1:32).

Consequently, the Corinthians should not have boasted of another’s sin but should have taken steps to prevent others from being defiled by associating with him. This is in accord with what is said in the Song of Solomon 2:2, As a lily among brambles, so is my love among maidens, on which a gloss comments, "He was not a good man who could not endure evil men."

When he says, Cleanse out the old leaven, he shows what should be done in the future. First, he presents the teaching; second, he gives the reason for it (in verse 7).

He says, therefore, because a little leaven corrupts the whole lump, cleanse out the old leaven. This means you should cleanse yourselves by casting out the "old leaven" from your midst—that is, the fornicator who returned to the old state of corruption by sinning: You are growing old in a foreign country, you are defiled with the dead . By cutting off one sinner, the whole group is cleansed; thus, when Judas left, the Lord said, Now is the Son of Man glorified (John 13:31). The "old leaven" can also be understood as the old error, as in Isaiah 26:3, The old error is passed away, or even the corruption of original sin: Seeing that you have put off the old nature with its practices (Colossians 3:9), for a person is cleansed by removing these things.

Second, he mentions the effect of this cleansing, saying, that you may be a new lump. Here, "lump" means a mixture of water and new flour before leaven is added. Therefore, once the leaven—that is, the sinner or the sin—is removed from the faithful, they become like a new lump, renewed in purity: Your youth is renewed like the eagle’s (Psalms 103:5); Be renewed in the spirit of your minds (Ephesians 4:23). Third, he mentions the basis for this cleansing when he says, as you really are unleavened—that is, without the leaven of sin. In this sense, the Lord says, Beware of the leaven of the Pharisees and Sadducees (Matthew 16:6).

When he says, For Christ our paschal lamb has been sacrificed, he gives the reason for what he had said. The reason why the faithful should be unleavened is taken from the mystery of Christ’s passion. First, he mentions the mystery; second, he draws his conclusion from it (in verse 8).

Regarding the mystery, it should be noted that the foremost sacrament of the Old Law was the Paschal lamb. As commanded in Exodus chapter 11, it was sacrificed by the whole multitude of the children of Israel to commemorate the event in which the angel, striking the firstborn in Egypt, passed over the homes of the Jews, whose doorposts were smeared with the blood of a lamb. The word "Pasch" (Passover) is derived from this event: It is the Lord’s Passover (Exodus 12:11). By virtue of this blessing, the people passed through the Red Sea (Exodus 14:15 and following). This lamb was a figure of the innocent Christ, of whom it is said, Behold the Lamb of God (John 1:36).

Therefore, just as that lamb was slain by the children of Israel so that God’s people could be delivered from the destroying angel and, after being freed from slavery in Egypt, pass through the Red Sea, so Christ was slain by the children of Israel so that God’s people might be delivered from the devil's attacks by His blood and from the slavery of sin by baptism, as if passing through the Red Sea. That lamb was called the Pasch of the Jews because it was sacrificed as a sign of the passing over; thus the disciples ask, Where do you wish us to prepare for you to eat the Passover? (Matthew 26:17), that is, the Paschal lamb.

Therefore, the Apostle says that you ought to be unleavened because, just as the Passover of the old people was the sacrificed lamb, so our Passover—that of the new people—is the sacrificed Christ. His sacrifice deserves the name "Pasch" both because of what the word means in Hebrew (namely, "passage") and what it means in Greek (namely, "passion"), for Christ passed from this world to the Father by means of His passion, in which He was sacrificed (John 13:1).

When he says, Let us, therefore, celebrate the festival, he draws his conclusion. To understand this, it should be noted that the Paschal lamb, after being sacrificed, was eaten with unleavened bread. Therefore, just as the Paschal lamb was a figure of our sacrificed Passover, so the observance of the new Passover should conform to the old. Accordingly, because the sacrificed Christ is our Passover, let us celebrate the festival by eating Christ not only sacramentally—Unless you eat the flesh of the Son of Man and drink his blood, you have no life in you (John 6:54)—but also spiritually by savoring His wisdom: Those who eat me will hunger for more, and those who drink me will thirst for more . And we should do so with spiritual joy: with glad shouts and songs of thanksgiving, a multitude keeping festival (Psalms 42:4).

He then describes how to celebrate by conforming the reality to the symbol, saying, not with the old leaven, the leaven of malice and evil. For it was commanded in Exodus chapter 12 that no leaven be found in the homes of those eating the Paschal lamb. Leaven involves both oldness and corruption. Thus, the removal of leaven could first mean the removal of the obligation to observe the precepts of the Old Law, which was rendered obsolete by the passion of Christ: The new coming on, you shall cast away the old (Leviticus 26:10).

Second, the removal of leaven could mean the removal of the corruption of sin, as we said above. In this sense, he says, nor with the leaven of malice and evil. Here, "malice" would refer to perverse actions—Casting away all uncleanness and abundance of malice (James 1:21)—and "wickedness" to crafty mischief: When he speaks graciously, believe him not, for there are seven mischiefs in his heart (Proverbs 26:25). Alternatively, according to a gloss, when he says, not with the old leaven, he refers to sin in general; but in adding, nor with the leaven of malice and evil, he becomes more specific, because malice refers to sin committed against oneself, and wickedness refers to sin against someone else.

Therefore, having set aside the improper way to celebrate, he describes the proper way when he continues, but with the unleavened bread of sincerity and truth. This means celebrating in sincerity and truth, which are symbolized by unleavened bread. Here, sincerity is contrasted with the corruption of sin, which he referred to as the leaven of malice and evil. "Sincere" means without corruption; thus in 2 Corinthians 2:17 he says, We do not adulterate the word of God but with sincerity in Christ we speak.

Truth, however, is contrasted with the symbols of the Old Law, as it says in John 1:17: Truth and grace came by Jesus Christ. This is because we should celebrate the true Passover in reality and not in symbols. Thus, according to a gloss, sincerity is understood as innocence from vice or newness of life, while truth is understood as the righteousness of good works or the directness that excludes deception.