The historicity of Christianity is very important. The danger of liberal theologies is their denial of historicity of Christianity.
The power of Christianity is based in the historical nature of it. The fall of Adam and the need for a second Adam to right the wrong of the first Adam are not some vague principles, they are realities.
The cross is not a principle to live by. The cross is something to have faith in. Jesus isn't primarily a moral teacher rather he is the object of faith.
There are many principles to learn in the cross and the teaching of Jesus and they are all of great importance. But the cross and teaching of Jesus can never be viewed simply moralistically without first coming to faith in the person of Jesus.
All the moral teaching in Christianity is based on the nature of God. First we must always learn who God is and what he has done. Then we can move onto moral thought.
Starting with the moral Christian teachings of Christianity is to reverse the proper order of theology, since the moral nature of Christianity is based on having a supernatural help by which one can live.
Faith proceeds works in salvation and works are only possible after faith.