What a person accepts as ultimate premises … shape(s) everything that follows. If you press any set of ideas back far enough, eventually you reach some starting point. Something has to be taken as self-existent - the ultimate reality and source of everything else. There’s no reasons for it to exist; it just “is”. For the materialist, the ultimate reality is matter, and everything is reduced t material constituents. For the pantheist, the ultimate reality is a spiritual force or substratum, and the goal of meditation is to reconnect with that spiritual oneness. For the doctrinaire Darwinist, biology is ultimate, and everything, even religion and morality, is reduced to a product of Darwinian processes. For the empiricist, all knowledge is traceable ultimately to sense data, and anything not known by sensation is unreal.
And so on. Every system of though 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 it, 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 is why the “Bible writers always address their reader as though they already believe in God or some God surrogate,” explains philosopher Roy Clouse. Faith is a universal human function, and if it is not directed toward God it will be directed toward something else….
In short, it is not as though Christians have faith, while secularists base heir convictions purely on facts and reason. Secularism itself is based on ultimate beliefs, just as much as Christianity is. Some part of creation - usually matter or nature - functions in the role of the divine. So the questions is not which view is religious and which is purely rational; the question is which is true and which is false.
This is what Augustine meant by his image of two cities. Ever since the Fall, the human race has been divided into two distinct groups - those who follow God and submit their minds to His truth, and those who set up an idol of some kind and then organize their thinking to rationalize their worship of that idol. Over time, as people’s ultimate commitments shape the choices they make, their perspective is inevitably molded to support those choices. A false god leads to the formation of a false worldview.
This is why Christians cannot complacently abandon so-called secular subject areas to non-believers - just so long as they grant us some restricted sacred area where we are free to sing hymns and read the Bible. Instead we must identify and critique the dominant intellectual idols, and then construct biblically based alternatives.
From "Total Truth: Liberating Christianity from Its Cultural Captivity", p. 41-42