In his Christian Theology: An Introduction Alister McGrath discusses the taxonomy of “natural heresies” outlined by Schleiermacher in The Christian Faith (147-49). Here is the gist of the argument.
1. The essence or basic principle of Christianity is that God has redeemed us through Jesus Christ.
2. The rejection of this principle is the rejection of Christianity itself. “In other words, to deny that God has redeemed us through Jesus Christ is to deny the most fundamental truth claim which the Christian faith dares to make.”