Yet not even Titus, who was with me, was compelled to be circumcised, even though he was a Greek. This matter arose because some false believers had infiltrated our ranks to spy on the freedom we have in Christ Jesus and to make us slaves. We did not give in to them for a moment, so that the truth of the gospel might be preserved for you.
In verses 1 and 2, Paul seemed to be questioning if he were doing the right thing, preaching the right gospel. I understand this, because the gospel that Paul was preaching was counter-intuitive. It just makes sense that we owe God, and that we should have to do something to pay that debt. We should desire to earn God's favor, because we are not perfect. There must be some element of the law that we can still cling to.
But not even Titus was compelled to be circumcised. If the Jews were going to keep any element of the law in place, it would seem circumcision would be a logical choice. It was instituted by God and labeled them, set them apart. Titus was a Greek, not a Jew. One would think that he would want ot make sure that he was safe, that he was a part of the "in" group. Why not be circumcised, if for no other reason than to play it safe? But Titus was confident that not only was no such action needed, it would be a hindrance to his faith. This was a part of hte freedom that Titus had found in Christ. Not a freedom to do anything that he wanted, but a freedom that meant he was not bound to the fruitless requirements of a legalistic system that couldn't fix the problem of sin.
Others observed this, and couldn't comprehend it. Or they simply did not want to give up the control that it gave to them. Sure, Christ saves, but they wanted the debt and allegiance of others to continue. Remember the later church's idea of indulgences? Same principle. You can be free in Christ! Now follow us in obedience to these laws and freedom will be yours.
Our freedom in Christ does free us from the law, but it does so in order that our service to Christ will be a response of love that honors him. That is the truth that Paul wanted to preserve! No one suffered more for the cross than Paul, but he did it not out of obligation to a set of rules. He did it as a response to what Christ had done for him.