If they are religious people, then they will defend the assertion of a god. I think the proper question to ask is, if there were no holy scriptures, would theism exist as we understand it today, would it exist at all, or would it be in some different from, unrecognizable to us?
Many civilizations through out time have worshiped a deity of some sort. Why? I can't tell you with certainty, but I would think that man in general feels someone had a hand in creating him. Should we need to rush out and join a religion? I don't think so. I myself don't get caught up in any religion, but I can't disprove a god. I'm not sure I buy the big bang theory either.
People can still talk and remember things they would have read in the scriptures, or heard from other, saintly people. It wouldn't be so hard to get the population to keep believing in God without the scriptures. It may not be so widespread, but it will be there. Though this later point is debatable as well.
In any case, people will just create more scriptures based on the experiences of the author, or authors. Things like this happen at the present anyway where people have mystical experiences and write a book about it, and people will believe that these people had these experiences.
You will never get rid of the belief in God in society.
All religions that have holy scriptures draw all support in their belief of a god from said scriptures. Without these, at most they would become Hume or Lockeian Deists who believe in a form of "love" or "hope" that is universal and equate that to god. As far as a traditional Abrahamic god though, even that concept would seem unattractive to those inclined to that.
Holy scriptures such as the bible, qou'ran etc. seems to be the basis of what - we in the present - use as a connection to the foundation of belief in different religions.
Yet, most of the time, I hear that people "feel" god's presence or witness miracles. The bible, for example, is undoubtedly written by humans. Therefore, without the bible, no one today would have any knowledge of god and his messages to humanity. Although the bible contains some references to historical locations (if there are any), there is no verifiable proof of that the events really occurred.
Nonetheless, if constructed symbols wouldn't had been written down in favor of our understanding, would people still be certain that a god exists?