Whoever will say to the mountain be thou removed, do not doubt and believe in his heart will have whatever he says
For with the heart man believes unto righteousness and with the mouth confessions are made.
We don't believe with our physical heart, that would just be believing with your body. When someone says the heart of an object, it refers to the very center or core. The center of man's core is his heart,his spirit. Believing with my heart refers to thinking and believing with the very core of my being.
Circumcision is that of the heart, not of the body. So there is an outward man who could be Jewish, but an inward Jew based on the cutting of the heart/spirit.
God is a spirit and if we worship Him, it must be done in spirit and in truth. Done from the heart
The Inward Man
2 Corinthians 4:16
Though the outward man (body) perish, there is an inward man (spirit) who is being renewed daily.
The problem in the Body of Christ is believing. Believers are having a hard time believing.