Romans 2:17-29 - It's not about foreskins!
The Jews had God's law through Moses (the 10 commandments etc) but Paul says this isn't enough to make them right with God (unless they're keeping it perfectly, which is impossible of course). Many of the Jews apparently thought that possessing the law was as good as keeping it but it's not or that God chose them because they're better than anyone else, which isn't true either. Paul's logic is: 'If you know God's law so well, you should also know that you're guilty of breaking it.' He even says that they've become so ungodly that they're the reason non-Jews mock God (v24). The same is true today. Some religious people doing 'religious stuff' have so much pride that rather than pointing people to Jesus, they actually push people away from him.
Paul takes the example of circumcision and explains that it's only a sign of something. As if cutting off your foreskin could ever be the thing that makes someone right with God! Ridiculous. It was only ever meant to be an outward sign of an inward reality. Like a wedding ring doesn't make people love each other but it's only a sign that they do. And just as it's possible to wear a wedding ring and not love your husband or wife, it's possible (and it should be stating the obvious really) to cut your foreskin off and not be right with God. In the same way, just as it's possible to be married and not wear the rings, it's possible to be right with God and not be circumcised. Sounds obvious doesn't it? But to a lot of Jews at that time, they thought their religion was what saved them instead of God. And people still make the same mistake today.
So it's not about foreskins and it's not about religion. They're not bad, it's just that people miss the point when they do the religious physical signs without the inward reality of knowing God. Being a Christian is not primarily about Bibles, churches and praying, though they ought to be useful. Being a Christian is first and foremost about being saved by Jesus and knowing God personally. So breathe a sigh of relief and throw your flint knife away!