Luke 12:1 In the meantime, when so many thousands of the people had gathered together that they were trampling one another, he began to say to his disciples first, "Beware of the leaven of the Pharisees, which is hypocrisy (hupokrisis i.e. pretension/pretending- hupokrite?s-literally an actor a stage player).
Galatians 5:9 A little leaven leavens the whole lump. (Note that Paul's use of the same statement in 1 Cor 5:1-8 deals specifically with a specific sin in a matter of sexual immorality)
The use of the word leaven as metaphor is found many times throughout scripture. Most commonly, the term "leaven" is thought to be used exclusively as a metaphor for sin. This is simply not so (See Luke 13:20-21)
As always, context determines meaning. In the above cited passages,both Jesus and Paul are using the term in conjunction with the same activity.The Lord says that the leaven of the pharisees/legalists is hypocrisy.Paul says that a little leaven leavens the whole lump. He is speaking of the leaven of the legalist/pharisees which permeates throughout that entire lump of dough.This dough representing both individual man and church,as well as the doctrines of grace, the gospel based in the scriptures, which is the Word of God.
The leaven written of by Paul is that very same hypocrisy spoken of by Christ. (Matthew 23)
The false doctrine of justification,sanctification,righteousness,i.e. salvation by works of the Law,any code, is the hypocritical leaven of the pharisees,or in today's church, the neo-pharisaical judaizers.
Salvation is a grace/faith issue, predicated upon God's sovereign (pre-creation) decree of election in Christ of all who will believe. Paul writes in Romans 14:23b; "For whatever (everything) does not proceed from faith is sin."
So then this hypocritical, man centered leaven is also sin, as it does not proceed from faith.
The "dough" is spoken of by Paul in several of his letters;
22 For Jews demand signs and Greeks seek wisdom,
23 but we preach Christ crucified a stumbling block to Jews and folly to Gentiles,
24 but to those who are called, both Jews and Greeks, Christ the power of God and the wisdom of God.
1 And I, when I came to you, brothers, did not come proclaiming to you the testimony of God with lofty speech or wisdom.
2 For I decided to know nothing among you except Jesus Christ and him crucified.
3 And I was with you in weakness and in fear and much trembling,
4 and my speech and my message were not in plausible words of wisdom, but in demonstration of the Spirit and of power,
5 that your faith might not rest in the wisdom of men but in the power of God.
6 Yet among the mature we do impart wisdom, although it is not a wisdom of this age or of the rulers of this age, who are doomed to pass away.
7 But we impart a secret and hidden wisdom of God, which God decreed before the ages for our glory.
8 None of the rulers of this age understood this, for if they had, they would not have crucified the Lord of glory.
19 For through the law I died to the law, so that I might live to God.
20 I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.
21 I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose.
Paul also gives us an excellent description of those who would try and work the leaven into the lump;
12 It is those who want to make a good showing in the flesh who would force you to be circumcised, and only in order that they may not be persecuted for the cross of Christ.
13 For even those who are circumcised do not themselves keep the law, but they desire to have you circumcised that they may boast in your flesh.
To this Paul proclaims the Lord's "recipe" for the making of the true unleavened bread;
14 But far be it from me to boast except in the cross of our Lord Jesus Christ, by which the world has been crucified to me, and I to the world.
15 For neither circumcision counts for anything, nor uncircumcision, but a new creation.
16 And as for all who walk by this rule, peace and mercy be upon them, and upon the Israel of God.( 1Cor 10:17;John 6:50-51)
Today our admonition to the saints is the same as in Paul's day and must be spoken with the same urgency;
1 O foolish Galatians! Who has bewitched you? It was before your eyes that Jesus Christ was publicly portrayed as crucified.
The gospel is defined as salvation wrought in and through the obedient work of Christ alone. The only work which saves. Any teaching that attempts to add man's participation into the mix is leaven,which has its' origins in the tree of the knowledge of good and evil. (Genesis 3:5 and 3:22)
Salvation is God's sovereign domain, man has no say in the matter. That which produces doctrinal hot air,(the cause of the lump becoming "puffed up") is the hypocrisy of those who would pretend to perform the work of salvation, thus becoming their own god.
22 Then the LORD God said, "Behold, the man has become like one of us in knowing good and evil. Now, lest he reach out his hand and take also of the tree of life and eat, and live forever--"
Man will never be allowed to touch the "tree of life" of his own volition.
Beware the leaven of the pharisees.
Soli Deo Gloria