Theists argue theism on the basis that a Universe that follows "laws" implies a "law giver" or creator, and our ability to comprehend those laws is further evidence of the divine purpose of their rational universe...
A rational universe is, of course, a universe which is held to firm laws. Though these laws may not always be properly understood, they exist. That is what it means for a universe to be rational.
Maybe its time to ditch the word "Law" from the the scientific lexicon. There's no "Law of Gravity" - and its an 18th Century concept that looks increasingly suspect. What we have are "models" of various aspects of the Universe, and insofar as the models continue to generate useful predictions, we continue to support them. When they start failing, we refine the model to the point where it might have to be replaced.
Our actual legal systems (at least the Common Law variety) follows this same pattern - these "Laws" reflect transient understandings of how society or the Universe works. This has nothing to do with "immutable" laws drawn from the Bible or any other religious source - and therefore carries no implication of a "law giver"
"Irrational" is the state of not being "rational." If there was a universe which was not rational, it would, by default, be irrational. "Irrational" is essentially the term used to designate things which are apparently impossible. "Impossible" is just another word for what cannot exist or transpire. An irrational universe, therefore, would be defined as a nonexistent universe. It is clear that nothing could possibly exist irrationally, as the very state of existing requires that there is some set of laws which allow for existence.
The universe, of course, exists. This means that the universe is, of course, rational.
The scientist has a working hypothesis that the universe can be understood by the use of reason. In this sense only it is 'a rational universe'. This is an epistemological claim.
The theist/deist on the other hand uses the phrase 'rational universe' to imply something (though that something is rather ill-defined) about the universe's nature or being, about its fabric. It is an ontological claim.
The theist claim that a universe's rational nature (ie its fabric) reflects and is caused and upheld by the rational nature of God (whatever that may mean, given that an omniscient being already knows all conclusions and therefore never has to reason toward them). The universe's supposed rational nature (in the ontological sense) only is so because it is given by God.
This argument does not work when the differnt of meanings of "rationality" described are revealed. Also, believing we may be able to understand some phenomenon by use of reason does not mean the phenomenon must be rational in its nature. Belief that we can understand black holes, for example, does not make black holes rational entities. In fact, the only 'things' which it is strictly correct to describe as being rational in their nature are the rules of logic and mathematics.
The point is illustrated by the claim that humans are rational beings. This, properly understood, is simply a claim that (some of the time) we use our brains to reason things out. But it is often misunderstood to be saying something more, ie that our very nature is 'rational'. This misunderstanding is often said to support the notion that we are made in the image of God, but is clearly untrue.
The universe shows no sign of being governed by a personal will or conscious intelligence. The impressive "order" we see is mathematical, impersonal and amoral.
I hope the above dialogue, edited from here, qualilfies as a good argument against a creator god.