For brief and clear definitions of basic theological words, turn to Baker's Dictionary of Theology. It's one of the best reference tools available and an essential help for reading Christian books. The quality and reliability of its content comes from renowned authors such as F. F. Bruce, G. H. Clark, F. W. Danker, E. E. Ellis, P. K. Jewett, G. E. Ladd, W. S. LaSor, J. Murray, J. I. Packer, C. van Til and many others.
