What is Christianity?

Shedd says:

Christianity, in the last analysis, is Trinitarianism. Take out of the New Testament the persons of the Father, the Son, and the Holy Spirit, and there is no God left. Take out of the Christian consciousness the thoughts and affections that relate to the Father, the Son, and the Holy Spirit, and there is no Christian consciousness left. The Trinity is the constitutive idea of the evangelical theology, and the formative idea of the evangelical experience. The immensity of the doctrine makes it of necessity a mystery; but a mystery which like night enfolds in its unfathomed depths the bright stars--points of light, compared with which there is no light so keen and so glittering. Mysterious as it is, the Trinity of Divine Revelation is the doctrine that holds in it all the hope of man; for it holds within it the infinite pity of the Incarnation and the infinite mercy of the Redemption.

Introduction to Augustine, On The Trinity, p11, Fig Books edition

http://mb-soft.com/believe/txue/august40.htm