Paul gives us a clear description of every true believer in Philippians 3:3. False religion is outward and carnal. True religion is inward and spiritual. False religion emphasizes things. True religion is a Person. False religion looks to and rests in what man has done. True religion looks to Christ and rests in what he has done.
True religion is an effectual operation of divine grace in the heart, "We are the circumcision". (Col. 2:10-12) Circumcision in the Old Testament was typical of that work of the Holy Spirit in a man's heart that we call the new birth. It was a mark of distinction. It was painful. It was cleansing. It was separating. It was permanent.
True religion is the spiritual, heart worship of God, "Which worship God in the spirit." Worship is not a physical exercise. It is a matter of the heart. (I Sam. 16:7; John 4:24; Isa. 66:1-4)
True religion rejoices in Christ. (Jer. 9:23-24; I Cor. 1:30-31) The word "rejoice" has the idea of confident joy, or such confidence and faith as produces joy. We who believe, "Rejoice in Christ Jesus", in His Person, in His Work, in His Word, in His Will, in His Providence, in His Promise, and in His Love.
Finally this verse tells us that true religion has no confidence in the flesh. Our confidence is in Christ, not in ourselves. We have complete confidence in him. But we have no confidence in the flesh. Yes, true religion is of the heart!