7. This is, then, wise counsel and serious admonition, that faithful guard be maintained against the infusion or introduction into doctrine of what is false, whether it pertains to works or faith. The Word of God, faith and conscience are very delicate things. The old proverb says: "Non patitur jocum fama, fides, oculus;"—Good reputation, faith and the eye—these three will bear no jest.
Just as good wine or precious medicines are corrupted by a single drop of poison or other impurity, and the purer they are, the more readily defiled and poisoned; so, also, God's Word and his cause will bear absolutely no alloy. God's truth must be perfectly pure and clear, or else, it is corrupt and unprofitable. And the worst feature of the matter is, the sway and intrenchment of evil is so strong that it cannot be removed; just as leaven, however small the quantity, added to the lump of dough, soon penetrates and sours the whole lump, while it is impossible to arrest its influence or once more to sweeten the dough.
8. The proposal of certain wise minds to mediate, and effect a compromise, between us and our opponents of the Papacy, is wrong and useless. They would permit preaching of the Gospel but at the same time retain the Papistical abuses, advocating that these errors be not all censured and rejected, because of the weak; and that for the sake of peace and unity we should somehow moderate and restrict our demands, each party being ready to yield to the other and patiently bear with it. While in such case no perfect purity can be claimed to exist, the situation can be made endurable if discretion is used and trouble is taken to explain.
Nay, not so! For, as you hear, Paul would not mix even a small quantity of leaven with the pure lump, and God himself has urgently forbidden it. The slight alloy would thoroughly penetrate and corrupt the whole. Where human additions are made to the Gospel doctrine in but a single point, the injury is done; truth is obscured and souls are led astray. Therefore, such mixture, such patchwork, in doctrine is not to be tolerated. As Christ teaches (Mt 9, 16), we must not put new cloth upon an old garment.
9. Nor may we in our works and in our daily life tolerate the yielding to the wantonness of the flesh and at the same time 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. Paul elsewhere warns (1 Cor 6, 9-10): "Be not deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with men, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God." Again (Gal 5, 19-21): "The works of the flesh are manifest ... of which I forewarn you, even as I did forewarn you, that they who practice such things shall not inherit the kingdom of God."
10. Warrant is given here likewise for censuring and restraining the rash individuals who assert that men should not be terrified by the Law, nor surrendered to Satan. No! it is our duty to teach men to purge out the old leaven; we must tell them they are not Christians, but devoid of the faith, when they yield to the wantonness of the flesh and wilfully persevere in sin against the warning of conscience. We should teach that such sins are so much the more vicious and damnable when practiced under the name of the Gospel, under cover of Christian liberty; for that is despising and blaspheming the name of Christ and the Gospel: and therefore such conduct must be positively renounced and purged out, as irreconcilable with faith and a good conscience.
"Purge out the old leaven, that ye may be a new lump, even as ye are unleavened."
11. If we are to be a new, sweet lump, Paul says, we must purge out the old leaven. For, as stated, a nature renewed by faith and Christianity will not admit of our living as we did when devoid of faith and in sin, under the influence of an evil conscience. We cannot consistently be "a new lump" and partake of the Passover, and at the same time permit the old leaven to remain: for if the latter be not purged out, the whole lump will be leavened and corrupted; our previous sinful nature will again have supremacy and overthrow the faith, the holiness upon which we have entered and a good conscience.
12. Paul does not here speak of leaven in general; he commands to purge out the "old leaven," implying there may be good leaven. Doubtless he is influenced by respect for the words of the Lord Christ where (Mt 13, 33) he likens the kingdom of heaven also to leaven. In this latter case leaven cannot be bad in quality; rather, the object in mixing it with the lump is to produce good, new bread. Reference is to the Word of God, or the preaching of the Gospel, whereby we are incorporated into the kingdom of Christ, or the Christian Church. Though the Gospel appears to be mean, is despicable and objectionable to the world, yet such is its power that wherever introduced it spreads, finding disciples in whom it works; it transforms them, giving to them its own properties, even as leaven imparts its powers to the dough and causes it to rise.