Romans 2:29 "...circumcision is circumcision of the heart, by the Spirit..."
Paul is giving it pretty good to some of the Judaizers claiming that they are hypocrites by not actually practicing what they preach. He claims you aren't a Jew just because you are circumcised or because on the "outside" you are Jewish. Being one of God's children comes from something on the inside...a circumcision of their hearts.
What is on the outside rarely matters. I can look good and fake it with the best of them. I know all the Christian customs, lingo, traditions, and motions. I know when to stand up, sit down, bow my head, say Amen, and clap along. But none of that really concerns God. He cares about what is on the inside of me. What good is it if I show up to church each Sunday, know all the lyrics, and have all the scriptures used by the pastor that day memorized, but don't love people or forgive them or serve them? Too much of my day is focused on externals. I need to continually challenge my heart, my soul, and who I really am on the inside. How can I become more like Jesus internally today?
Dear Jesus, help me make sure the inside of my cup is clean and not just the outside. Help me become more like you today. Amen.