Christianity is a major global faith that believes in a God who created the universe and who saved humanity by entering history in the person of Jesus Christ and dying for humanity’s sins. Its core doctrines are derived from the biblical canon of the Old and New Testaments (collectively referred to as the Bible). It is the largest world religion and is diverse in its practices and expressions, but instead of highlighting the nuances of particular Christian denominations, Keller focuses on the common ground of core doctrines held by most Christians. Keller often prefers to describe Christianity more in terms of a relationship between God and humanity than as a religion, and he regularly makes distinctions between Christianity’s fundamental vision of reality and that of other world religions.

The term “church” is used in two distinct ways in The Reason for God, as in most Christian books. It can refer either to a particular local congregation or to the full collective of all Christian believers from every time and place. Since Keller is a pastor, many of his references to church have to do with his experience of ministry in his own congregation, Redeemer Presbyterian Church (See: Key Figures).