Despite the OT having the Law for the Hebrew (Jewish) people, it doesn't exactly give the author (Moses, except at the end, which was probably Joshua or another contemporary) all that much political power. It's also more of a historical document than it is a political one. Moses was already the leader of the Hebrew people by leading them out of slavery, so he doesn't have much of a reason to establish a government and give himself power through it. Actually, Moses never gets to see a country born at all because the Israelites were in a desert for forty years, scared of attacking the Canaanites. So, TL;DR If Moses wrote the Torah for political power that he already had, he was really bad at it. In fact, the system they set up ended up with the Hebrew leaders [i]losing[/i] power because it established a system of judges to control the land (similar to our Supreme Court). So to summarize, the Torah (first five books of the Bible) was written by a party that had no stake in the contents, so historians generally accept what was written as uninfluenced by a desire for political gain (especially since most of it isn't even political—it's a description of historical events).

Beyond that, the rest of the Old Testament actually has authorship with zero ability to be politically motivated by their writings. Most of them were persecuted or even killed for what was written, while most of it is historical writings recounting what was said or done by the Hebrew people. While there are commandments by God (mostly to Israel as a nation in laws and traditions rather than moral guidelines, though many of these do exist) in these writings, they aren't presented in a context that would suggest some random Hebrew wants to (or even could) influence the nation as a whole. And as for their history, they've certainly been questioned about whether or not they're accurate, but in fact, most ancient historians accept the reality of those events presented. A common objection is that because we know it was written by the Hebrews, we can contend that its recountings were biased in their favor. However, two of the main tenets historians use for analyzing the credibility of a source is whether A) The source would provide information that would be an embarrassment or a disgrace to themselves, and B) Whether they would provide information that would clearly damage their credibility. The ancient Hebrew historical writings fulfill both of these—the accounts of the Jewish people show that they did incredibly horrible things and failed over and over. The authors are often common people, such as farmers or those in poverty (though there are the writings of kings and scholars). Because of these reasons and other techniques, historians generally accept the events depicted in the Old Testament (on a historical, though not necessarily miraculous level). Before I talk about the validity of the New Testament, are there any questions about this or information you would like to dispute?