To me Paul is saying circumcise the heart. When you accept Jesus as your personal savior the Holy Spirit moves through you changing your heart, turning it into a heart of flesh. You forgive those that have done you wrong. You want to live a life that’s pleasing to God. The first thing that should happen when someone is truly saved is that you see a change in heart. The heart is where every word flows from. The heart is where every word is conceived before it comes out of your mouth. And if you’re not seeing or feeling a change of heart, you need to be praying, change my heart oh, Lord. Give me a heart of flesh. When you start praying this prayer you will see and feel the changes that God is making within you. You will feel a need to give to a ministry or support your local church. You will feel and see the need of others, and become selfless. But, God cannot use you until you accept the changes he’s making within you. Your heart is the stepping stone to being a kingdom builder.
Psalm 51:10 Create in me a pure heart, O God, and renew a steadfast spirit within me.
Proverbs 4:23 Above all else, guard your heart, for everything you do flows from it.
Ezekiel 36:26 I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh.