‘Your glorying is not good. Do you not know that a little leaven leavens the whole lump? Purge out the old leaven that you may be a new lump, even as you are unleavened. For our Passover has also been sacrificed, even Christ.'
The Corinthians were glorying in what they saw as their high spirituality and their tolerance, but Paul points out that they have no right to glory while there is open sin prevalent among them. A small amount of leaven will soon permeate and affect a whole lump of dough. In the same way a relatively small amount of fermenting sin is infectious, it will soon affect the whole church (compare Galatians 5:9 where the same point is made about false teaching, which is also in the background here).
So the people of God must rid themselves of sin, and especially cast out those guilty of open sin unless such sinners are ready to repent and put right what is wrong, and they must begin with this man who has sinned so grievously. But having purged him from the assembly they must also purge themselves within the assembly. Thus will they become like a new lump of dough that is unleavened, for they will have removed sin from among them.
This picture of leaven leavening bread is taken from the feast of the Passover and unleavened bread. There, before the feast began, all leaven had to be removed from the houses of the participants and a diligent search made to ensure none was left. So must Christians root out sin from within and among them. Paul was probably hoping for an instant revival, while practical enough to know that it might not happen like that. But that it did happen to some extent is suggested by 2 Corinthians 2:5-11.
Leaven consisted of old dough which had been allowed to ferment. It was then introduced into new dough with its leavening effect, causing the new dough to expand. The leaven would spread through the whole which was visibly affected. It was seen as a type of corruption. There would indeed come a time when the leaven had become too acidic and was unhealthy, thus the wise necessity for getting rid of all leaven once a year and starting again.
‘Christ our Passover.' The thought of leaven and unleavened bread leads on to the thought of Christ as the Passover lamb. Having cleared themselves of leaven the Passover would follow. So the reason why they should get rid of the leaven is because they know that the unblemished Lamb Himself has been sacrificed for us once for all (aorist) so that we might be cleansed from sin and partake of what is holy. This is why Christ died as a sacrifice, for the forgiveness and removal of sin. And God's people, ‘the church', must therefore be holy, set apart to Him in purity and righteousness, as they set their eyes, thoughts and hearts on Him. If the Lamb Who was sacrificed for us was unblemished and holy (that the Passover offering could not be eaten outside the dwelling established its essential holiness), with no bone broken, an indication of His complete perfection, so must we who benefit from His death, and from His sacrifice of Himself, and who partake of Him by faith, be concerned to be a holy ‘lump' free of all corrupting leaven.
Note how it is the connection with the sacrifice on the cross that ensures that all sin is dealt with. Because He has been sacrificed for us we can and should again be made clean (1 John 1:7-10) having set sin aside. In the light of that sacrifice all should recognise that the old leaven must be totally removed. No sin can be allowed to endure the presence of the crucified One. The word of the cross is the great purifier. No sin can be allowed to remain in its way.
‘Purge out the old leaven.' The fact that the ‘old leaven' is spoken of in such a way as to suggest it is different from the leaven of malice and wickedness has led some to see it as referring to the old doctrines of Judaism as incorporated into a form of Christianity by certain Teachers, which have to be done away with and rooted out (as in Galatians 5:9). They bring the wisdom/folly of the scribes which must be purged out (1 Corinthians 1:20). This might then be seen as especially spoken of those who ‘belong to Cephas'.
Or ‘the old leaven' is seen by others as something known to them and Paul, some defiling thing on which they disagree. For central to this passage here is the fact that Paul is speaking in the context of a case of gross immorality. Thus any doctrines in mind might be such as caused such immorality to be overlooked, that is, some form of lax doctrine which allows such behaviour, some form of antinomianism (lawlessness) that concentrated on spiritual gifts at the expense of morals. Thus the ‘old leaven' might point to the teaching of some of the ‘wisdom teachers' in the church which has resulted in sinful licence. But alternately it may refer to the gross sins and their contaminating influence which have to be put aside if they are to be restored to holiness.
‘That you may be a new lump even as you are unleavened.' Paul desires that the Corinthians become a ‘new lump of unleavened dough'. He wants all corruption removed. He wants them as it were to come back to the word of the cross through repentance and begin again, having been cleansed in the blood of Christ (1 John 1:7-10). He wants them to be renewed. This parallels his pleas elsewhere that Christians put off the old man and become ‘a new man' (Ephesians 4:22-24 compare Romans 6:11; Galatians 4:19), something which in one sense happens once for all, but in another sense has to be repeated (Galatians 4:19). He wants them not only to have newness of life but to walk in newness of life (Romans 6:4). They are to become a new pure lump through their connection with the Passover sacrifice.