see HERE how the New Testament revised the practise of circumcision from The Old Testament)
This is one I find interesting, because people seem to read it the wrong way. The verse is in the New Testament (so the Old Testament has ended, as that site says). Part of that verse says, "Circumcision availeth nothing." But it is only part of what it's saying. It is warning people not to think they're automatically clean/holy just because they've been circumcised. It's basically saying if their heart is wrong, being circumcised won't change that, or score them any "holy" points. (Not that it that it shouldn't be done - and not that it has no benefit.)