[Paul]has been reproving the Corinthians for their disposition to boast of the Gospel and of Christ while abusing such liberty unto unchastity and other sins. He admonishes them that, possessing the Gospel and having become Christians, they ought, as becomes Christians, to live according to the Gospel, avoiding everything not consistent with the faith and with Christian character--everything not befitting them as new creatures.
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But in this text he has reference more particularly to an erroneous idea concerning life and conduct. In this instance it is likewise true that, once the flesh be allowed any license, and liberty be abused...there is introduced a leaven which will speedily corrupt faith and conscience, and continue its work until Christ and the Gospel are lost. Such would have been the fate of the Corinthians had not Paul saved them from it by this epistle admonishing and urging them to purge out the leaven of license; for they had begun to practice great wantonness...
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Nor may we in our works and in our daily life tolerate the yielding to the wantonness of the flesh and at the same boast the Gospel of Christ, as did the Corinthians, who stirred up among themselves divisions and disorder, even to the extent of one marrying his stepmother. In such matters as these, Paul says, a little leaven leavens and ruins the whole lump--the entire Christian life. These two things are not consistent with each other: to hold to the Christian faith and to live after the wantonness of the flesh, in sins and vices condemned by the conscience.
excerpted from
Exhortation To Walk As Christians
A Sermon by Martin Luther; 1540
The entire sermon may be found here http://www.orlutheran.com/mlse1co5.html
An excellent read