From Nancy Pearcey’s Total Truth: Liberating Christianity from Its Cultural Captivity:
Every system of thought begins with some ultimate principle. If it does not begin with God, it will begin with some dimension of creation–the material, the spiritual, the biological, the empirical, or whatever. Some aspect of created reality will be ‘absolutized’ or put forth as the ground and source of everything else–the uncaused cause, the self-existent. To use religious language, this ultimate principle functions as the divine, if we define that term to mean the one thing upon which all else depends for existence. This starting assumption has to be accepted by faith, not by prior reasoning. (Otherwise it is not really the ultimate starting point for all reasoning–something else is, and we have to dig deeper and start there instead.
In this sense, we could say that every alternative to Christianity is a religion. It may not involve ritual or worship services, yet it identifies some principle or force in creation as the self-existent cause of everything else. Even nonbelievers hold to some ultimate ground of existence, which functions as an idol or false god. This why the ‘Bible writers always address their reader as though they already believe in God or some God surrogate,’ explains philosopher Roy Clouser. Faith is a universal human function, and if it is not directed toward God it will be directed toward something else.