Galatians 2:3-5

Yet not even Titus, who was with me, was compelled to be circumcised, even though he was a Greek.

(v3, NIV)

The issue of circumcision is brought up a number of times throughout the New Testament. One of the main problems the early church had was that some of them were teaching that you needed to be circumcised in order to be saved. This is an incorrect teaching that is addressed when the issue of circumcision is brought up in the New Testament.

Greeks were part of the gentiles. If you didn't already know, the gentiles are everybody who is not a Jew. It anybody was a promoter of circumcision, it a Jew. This is because they were given the law and circumcision is taught in the law of the Old Testament.

Paul points out that Titus was not compelled to be circumcised. As Paul is presenting the gospel as a means to test himself to ensure he is not running in vain, I assume that Titus and Barnabas are also doing the same thing. Since some people in the early church were influenced by the teaching of circumcision, Paul was probably afraid that Titus thought it was necessary to be circumcised.

This matter arose because some false brothers 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 remain with you.

(v4-5, NIV)

It is true that there exist false brothers among Christians. I'll bet that if you go into most any church and look hard enough, you can find at least one false brother. Take for example the parable of the weeds in Matthew 13. The weeds are sown among the wheat and it's hard to tell the wheat among the weeds. It is not until the harvest that they are separated. In the same way, there will be false brothers among us. However, it is hard to tell, so I don't think its right to go around calling people who for being a false brother. This is of course different from rebuking one another, but I digress.

These false brothers want to make them slaves. This is not in the sense that they are slavers and want them to become their slaves. Instead, they want them to become slaves of sin. These false brothers want to spread the teaching that circumcision is necessary for salvation or a similar teaching. This teaching is a gospel that is different from God's gospel and Paul knows this. Paul does not give into this false teaching so that he may be able to properly teach those he is going to spread the gospel to.