The very center and core of the whole Bible is the doctrine of the grace of God – J. Gresham Machen

“The very center and core of the whole Bible is the doctrine of the grace of God — the grace of God which depends not one whit upon anything that is in man, but is absolutely undeserved, resistless and sovereign. The theologians of the Church can be placed in an ascending scale according as they have grasped that one great central doctrine, that doctrine that gives consistency to all the rest; and Christian experience also depends for its depth and for its power upon the way in which that blessed doctrine is cherished in the depths of the heart. The center of the Bible, and the center of Christianity, is found in the grace of God; and the necessary corollary of the grace of God is salvation through faith alone.” 

– J. Gresham Machen, 

quoted in Ned B. Stonehouse, J. Gresham Machen: A Biographical Memoir (Grand Rapids, 1955), page 396.