The church is the new covenant community – “This cup is the new covenant in my blood” (Luke 22:20; 1 Corinthians 11:25) – we say when we take communion. The new covenant is the spiritual work of God to put His Spirit within us, write the law on our hearts, and cause us to walk in His statutes. It is a spiritually authentic community. Unlike the old covenant community it is defined by true spiritual life and faith. Having these things is what it means to belong to the Church. Therefore to give the sign of the covenant, baptism, to those who are merely children of the flesh and who give no evidence of new birth or the presence of the Spirit or the law written on their heart or of vital faith in Christ is to contradict the meaning of the new covenant community and to go backwards in redemptive history.
John Piper