the advantage of circumcision

Romans 3:1-7

Observe:
Now it seems like Paul is shifting to address the Gentiles (he speaks of the Jews in the third person instead of in the second person as he has done in the previous section).
Paul is contrasting God's character to human character and basically saying that God's character is seen more clearly when contrasted to human character but that doesn't get us off the hook from facing God's judgement.
Paul starts a list in v. 2 of all the advantages that a Jew has over a Gentile. However, he only gets to number one and then he interrupts himself (a classic Paul tactic). According to my study Bible, he doesn't pick up the list again until chapter 9.

Interpret:
First, Paul is saying that Jews have an advantage over Gentiles because they have been entrusted with the word of God. This advantage also carries responsibility with it - to live according to the revelation they have been given through the words of God.
Paul then goes on a bit of a side track by saying that even if some Jews do not believe the word of God, it does not change the fact that God is faithful. We tend to think of God being faithful to us but Paul says that he is faithful to his own character - his judgement of sin exhibits his faithfulness to himself (in other words, he will never act outside of his character).
Paul goes on to destroy an argument that people were making: God cannot be just when he judges us because his character is seen most clearly in contrast to ours (i.e. his faithfulness in contrast to our unfaithfulness). This is similar to an argument that people make today that God cannot be just in judging those who don't have access to the gospel of Christ. God is just and he will judge in righteousness (Paul argues this in the previous chapters).

Apply:
As a Christian living in a free country, born to Christian parents and having access to many different versions of the Bible in my first language, I have been entrusted with a huge responsibility before God to live up to and take care of his words. I have been given many of the same advantages that the Jews had in Paul's day. This means that I must read, study, treat with respect, teach and live out the words of Christ as they have been given to me and as they are taught to me by the Holy Spirit.
Thankfully I don't hear too much of the argument that Paul was fighting against (let's do bad so God's goodness can be clearly seen). I have heard a variation of it once from a friend of mine who was a terrible driver (passed on the shoulder, 1.5x the speed limit, etc.) who saide he couldn't wait to get pulled over so that he could show the police officer respect and humility and be an example of Christ to the officer. I think God would rather have my friend be an example of Christ in the way that he drove. This passage reminds me that I am to display God's faithfulness, justice, truthfulness, etc. not by contrasting it against my own unfaithfulness, etc. but by allowing the Spirit of God to produce the character of Christ in me. I should represent, not be an anti-example of, God's character.