Why am I a theist? I think that’s because language can be used for classification and division.
The word for God in Greek is θεός (theos) and the word used in the question is derived from this to classify someone as a believer in a god. Greek language and culture have also provided a great many things, from φιλοσοφία (philosophy), Δημοκρατία (democracy) along with a wealth of art, education, sports and ethical teaching.
For example, Aristotle could be considered one of the founding thinkers for the division and classification of things we find in the everyday world. However, Aristotle was far from being irreligious. If I quote R. Michael Olsen
Aristotle conceives of God as an unmoved mover, the primary cause responsible for the shapeliness of motion in the natural order, and as divine nous, the perfect actuality of thought thinking itself, which, as the epitome of substance, exercises its influence on natural beings as their final cause.These two aspects of God reflect the two defining aspects of Classical Greek Philosophy: the experience of the intelligibility of the natural order and the search for the first principle(s) responsible for its intelligibility.
Image courtesy NT Times.
As we can see from the example above, belief in God or gods has a rich tradition which provides the fabric within the tapestry of ideas derived from the ancient world that continue into the present.
Far from that world view being irrelevant, the way of looking at God as providing rationale for the intelligibility of the natural order is as relevant today as it was when first articulated.
The assumption that the natural order is intelligible without recourse to God or gods is an more modern phenomenon. That is to say, reliance on “empirical evidence” as the only source of truth flies in the face of many thousands of years of thinking and, when held without a degree of self awareness, risks being little more than unsupported assertion.
For instance, the problem of induction remains a problem even if one adopts the most hard line empiricist position. Indeed the more hard line the position is taken the more difficult it is to explain faith in causality, since any evidence used to support that belief is self-referential.
Personally, my beliefs are a result of faith. Something that is seen and observed would typically not be expected to require faith. The call for evidence is the very opposite of a call for faith, with the exception of the underlying assumption of the intelligibility of nature.
I try to draw the boundaries between what I believe, what I can determine by observation and what can be deduced. Faith is not bad in itself, since it becomes the motivator for achievement and enables accomplishment. An unstated faith or assumption however, can lead its owner in unknown directions.
Certainly, any personal experience, or indeed any other perception, spiritual or otherwise could be an illusion. Causality and the reliability of empirical data could themselves be an artefact of sheer random chance. Which is more of concern, statements of faith that are clearly articulated as such or assumptions and faith that are not articulated, but declared to be fact?
 Aristotle on God: Divine Nous as Unmoved Mover