Jesus warns us in Luke 12:1 about leaven: "Beware of the leaven of the Pharisees, which is hypocrisy." Throughout Matthew 23, Jesus lists a multitude of Pharisaical sins that could be grouped as legalistic externalism.
In Matthew 16:6, Jesus warns of the leaven of the Sadducees. The Sadducees' sins are not listed, but elsewhere we find they at least denied the supernatural and the resurrection of the dead (Acts 23:8). Jesus also warns of the leaven of Herod (Mark 8:15). Herod was involved in a great deal of lying in his political wheeling and dealing, abusing the power of his office, adultery, and general all-around worldliness.
Paul commands in I Corinthians 5:7-8:
Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.
Thus, in the New Testament leaven signifies wickedness and malice in contrast to sincerity and truth.
All of our offerings to God are mixed with some measure of sin. Has He made allowance for this in His instructions for the offerings? Yes.
No grain offering which you bring to the LORD shall be made with leaven, for you shall burn no leaven nor any honey in any offering to the LORD made by fire. As for the offering of the firstfruits, you shall offer them to the LORD, but they shall not be burned on the altar for a sweet aroma. (Leviticus 2:11-12)
Leviticus 23:17, 20 clarifies this:
You shall bring from your dwellings two wave loaves of two-tenths of an ephah. They shall be of fine flour; they shall be baked with leaven. They are the firstfruits to the LORD. . . . The priest shall wave them with the bread of the firstfruits as a wave offering before the LORD, with the two lambs. They shall be holy to the LORD for the priest.
This Pentecost offering is a meal offering. The loaves represent Christians accepted before God because of Jesus Christ. However, because the loaves contained leaven, symbolizing the reality of sin in our lives, they are waved before God and accepted but not burned on the altar, recognizing the presence of that sin.
Romans 7:14-20 makes a powerful statement on this:
For we know that the law is spiritual, but I am carnal, sold under sin. For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do. If, then, I do what I will not to do, I agree with the law that it is good. But now, it is no longer I who do it, but sin that dwells in me. For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find. For the good that I will to do, I do not do; but the evil I will not to do, that I practice. Now if I do what I will not to do, it is no longer I who do it, but sin that dwells in me.
No matter how much oil—the Holy Spirit—is poured out on us, it cannot completely counteract the corrupting effect of the leaven. We can control the flesh sufficiently so sin does not rule us, but sin is ever with us, and as long as we have human nature, that cannot be changed.
The only solution is that we must be changed—totally—and that is in our future, according to I Corinthians 15:50-52:
Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption. Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed—in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.
John W. Ritenbaugh
The Offerings of Leviticus (Part Three): The Meal Offering