The gospel is not just for unbelievers. It is for Christians too. You cannot relate to God in any other way. Put positively, the Good News of Christ’s cross is the only way of relating to God. It is the basis of His grace alone. We are not only saved by grace but we live by the grace of the Good News of Christ’s cross. That is why the cross is so important. It is the main thing, it is the center.
