REASON WORKS; BUT THE BEST CASE IS AESTHETIC:

The idea of natural theology is the idea of knowledge of God's existence and nature that is attainable through purely philosophical arguments, entirely independently of any special divine revelation.  (This is usually contrasted with revealed theology, which is knowledge of God's existence and nature attainable through some special divine revelation - for example, through a prophet sent by God whose veracity is backed by miracles.)  In Western philosophy, natural theology goes back to the very beginning, to the Greeks - and not just to Plato and Aristotle, but to the Pre-Socratics.  Arguably it begins with the second of them, Anaximander of Miletus (610 - 546 B.C.).

The Pre-Socratics inaugurated the search for what they called the archē of all things, where the term "archē" originally connoted either a beginning point or a position of authority.  An archē is a principle of order, and the search for the archē of all things is essentially the attempt to find an ultimate source and explanation for the order of the world.  Anaximander's predecessor Thales famously proposed water as the source from which all else derives.  His view seemed to be that the ordinary objects of our experience are all water in various configurations.  Perhaps he had in mind the idea that just as water can in everyday experience take on a liquid, solid, or gaseous form, so too the other objects of our experience are just further transformations of it (an idea analogous to Anaximenes' later proposal that all things are air in various forms).

However, and Anaximenes notwithstanding, the tradition largely and quickly moved beyond such crudely materialistic models.  Even those Pre-Socratics who took the archē to be in some way material came to see that it had to be radically unlike any of the objects of ordinary experience.  And as Lloyd Gerson notes in his book God and Greek Philosophy, the trajectory of the Greek tradition was toward locating the ultimate explanation of things in a single archē that exists of necessity.  The theistic implications of this line of thought are obvious, and some thinkers did indeed arrive at conceptions of the archē that would deeply influence the classical theist tradition - for example, Xenophanes' non-anthropomorphic philosophical monotheism, Parmenides' Being, Plato's Form of the Good, Aristotle's Prime Unmoved Mover, and Plotinus's One. 

Arguably we see something like a germ of classical theism already in Anaximander's notion of the apeiron as the source from which all else derives. 

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