But Paul refers here to old, inactive and worthless leaven. He means teachings, views, or manner of life resulting from the Old Adam, from flesh and blood, and destructive of the pure, new doctrine, or a nature renewed by Christianity. Later on he terms it the "leaven of malice and wickedness," and in the verse under consideration bids the Corinthians be a new, pure lump.
13. Note the apostle's peculiar words. He enjoins purging out the old leaven, assigning as reason the fact: Ye are a new and unleavened lump. By a new unleavened lump he means that faith which clings to Christ and believes in the forgiveness of sin through him; for he immediately speaks of our Passover: Christ, sacrificed for us. By this faith the Corinthians are now purified from the old leaven, the leaven of sin and an evil conscience, and have entered upon the new life; yet they are commanded to purge out the old leaven.
14. Now, how shall we explain the fact that he bids them purge out the old leaven that they may be a new lump, when at the same time he admits them to be unleavened and a new lump? How can these Corinthians be as true, unleavened wafers, or sweet dough, when they have yet to purge out the old leaven?
This is an instance of the Pauline and apostolic way of speaking concerning Christians and the kingdom of Christ; it shows us what the condition really is. It is a discipline wherein a new, Christian life is entered upon through faith in Christ the true Passover; hence, Easter is celebrated with sweet, unleavened bread. But at the same time something of the old life remains, which must be swept out, or purged away. However, this latter is not imputed, because faith and Christ are there, constantly toiling and striving to thoroughly purge out whatever uncleanness remains.
15. Through faith we have Christ and his purity perfectly conferred upon ourselves, and we are thus regarded pure; yet in our own personal nature we are not immediately made wholly pure, without sin or weakness. Much of the old leaven still remains, but it will be forgiven, not be imputed to us, if only we continue in faith and are occupied with purging out that remaining impurity.
This is Christ's thought when he says to his disciples (Jn 15, 3), "Already ye are clean because of the word which I have spoken unto you," and in the same connection he declares that the branches in him must be purged that they may bring forth more fruit. And to Peter—and to others—he says (Jn 13, 10), "He that is bathed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all." These passages, as is also stated elsewhere, teach that a Christian by faith lays hold upon the purity of Christ, for which reason he is also regarded pure and begins to make progress in purity; for faith brings the Holy Spirit, who works in man, enabling him to withstand and to subdue sin.
16. They are to be censured according to whose representations and views a Christian Church is to be advocated which should be in all respects without infirmity and defect, and who teach that, when perfection is not in evidence, there is no such thing as the Church of Christ nor as true Christians. Many erring spirits, especially strong pretenders to wisdom, and precocious, self-made saints, immediately become impatient at sight of any weakness in Christians who profess the Gospel faith; for their own dreams are of a Church without any imperfections, a thing impossible in this earthly life, even they themselves not being perfect.
17. Such, we must know, is the nature of Christ's office and dominion in his Church that though he really does instantaneously, through faith, confer upon us his purity, and by the Spirit transforms our hearts, yet the work of transformation and purification is not at once completed. Daily Christ works in us and purges us, to the end that we grow in purity daily. This work he carries on in us through the agency of the Word, admonishing, reproving, correcting and strengthening; as in the case of the Corinthians through the instrumentality of Paul. Christ also uses crosses and afflictions in effecting this end.
He did not come to toil, to suffer and to die because he expected to find pure and holy people. Purity and holiness for us he has acquired in his own person to perfection, inasmuch as he was without sin and perfectly pure from the moment he became man, and this purity and holiness he communicates to us in their flawless perfection in so far our faith clings to him. But to attain personal purity of such perfection requires a daily effort on the part of Christ, until the time shall have come that he has wrought in us a flawless perfection like his own.
So he has given us his Word and his Spirit to aid us in purging out the remaining old leaven, and in holding to our newly-begun purity instead of lapsing from it. We must retain the faith, the Spirit and Christ; and this, as before said, we cannot do if we give place to the old carnal disposition instead of resisting it.