The apostle Paul knew how to preach about Jesus without giving offense, but he refused to do so. In fact, he understood offensiveness as a characteristic of the true gospel. It’s a hallmark of the faith to be offensive to those who seek to be justified in the flesh.
And I, brethren, if I yet preach circumcision, why do I yet suffer persecution? then is the offence of the cross ceased.
Galatians 5:11
The letter to the Galatians was written to combat the spin that some teachers were putting on the gospel at the time. These teachers wanted to add a commandment to be circumcised to the requirements for salvation. (See also Acts 15:1ff)
It isn’t that commandments are such a problem. The Lord Jesus has charged his followers with many commandments; “Judge not, that ye be not judged,” or “Take no thought for your life, what ye shall eat, or what ye shall drink,” are examples. Like Jesus, his servants haven’t shied away from teaching us how to live godly lives. Peter talked for a long time in Acts 2 explaining to the new believers how different their lives needed to be from that of the society around them.
It isn’t that the commandment of the false teachers was so hard either. “Be ye therefore perfect, even as your Father which is in heaven is perfect” is hard. “Love one another, as I have loved you” is hard. In contrast circumcision is much easier.
And it’s not like it’s a big change, you might be tempted to think. The false teachers didn’t want to the Galatians to set aside all the teachings of Christ. The false teachers say all the same things that the apostles said. They just add one little thing. What could be the harm? Maybe we should do it just to be safe. And even if we don’t really have to do it, it obviously could bring greater unity. Why be divided from anyone who professes to love our lord?
Our stubbornness on this point is highly offensive. Is the offense itself reason enough to change? What does the apostle say?
A little leaven leaveneth the whole lump.
Galatians 5:9
What is leaven? It is a sourdough starter. It is flour and water which has been left un-refrigerated until yeast and lactobacillus bacteria get established in it. When a baker wants to make a loaf of bread, he mixes the sourdough starter into a new batch of bread dough. The yeast and bacteria go work as soon as they are introduced to the loaf. The quickly multiply and turn the whole thing into sourdough.

This is leaven.
In modern terms we might say that the loaf is infected with yeast and bacteria. That certainly sounds unappetizing, but this process was the normal one used for producing bread until just a couple hundred years ago.
For the apostle Paul, the commandment to be circumcised was an infectious disease that would poison the whole gospel, even though it was just one little thing.
Now I bring this up not because I know of any strong push for circumcision in America today. In a sense, circumcision is only an example of what might threaten to destroy the gospel. It isn’t enough for us to make a mental note “no circumcision, OK, got it.” We need to see the principle at work. We want to understand the how and why, so that no other leaven poisons the loaf.
The apostles makes a shocking statement towards the beginning of Galatians 5.
Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing. For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
Galatians 5:2-3
How can you make Christ’s sacrifice ineffective? Get circumcised. What?! Why would that make Christ’s sacrifice useless to me? Because if you are going to be circumcised, you have to keep the whole law.
Now would be a good time to time to bring up the fact that Paul moves effortlessly between different meanings of the word “law.” Sometimes he speaks positively about “the law” and sometimes negatively. For example, look at verse 14:
For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.
Galatians 5:14
If the whole law is loving your neighbor, and we are commanded to love our neighbor, then what could be wrong with being required to keep the whole law, if it’s all wrapped up in the commandment to love anyway?
“Not so fast,” someone might say. “Jesus said the law was wrapped up in two commandments, the first being to love God, and the second being to love your neighbor.”
That’s exactly right. In verse 14 the apostle is using the word “law” to mean the moral law that governs our right relationships with one another. The apostle distinguishes between multiple types of law and we must do the same.
No doubt there was a great deal of effort put in by the apostle to distinguish so easily between the two. He was a Pharisee before Jesus appeared to him. He was fanatic for the law of Moses and the traditions of Judaism. When Jesus appeared to him and proved that he was the Christ, Paul had a lot of unlearning to do. This is because for all his knowledge of the law he had never really understood it before. He thought that the law was intended to justify men, to make them right with God. In truth, that is the work of Christ alone, and the law of Moses was given to teach us about that truth.
If you don’t see Christ in the law, you don’t see it at all.
Photo of freshly baked bread by Flickr.com user “FotoDawg” (http://www.flickr.com/photos/fotodawg/111052838/) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)%5D, via Wikimedia Commons
Photo of leaven by User:Stephane8888 (Stephane8888 Wiktionnaire) (http://commons.wikimedia.org/wiki/Main Page) [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5) or CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)%5D, via Wikimedia Commons