The topic of the "truth or fiction" of religion presupposes 1.) that there is a "truth" we can know and 2.) that fiction is not "truth." Let's start with the second supposition, as it's the easier to wrestle with. Fiction has two senses, but primarily it is a literary form. The fact that the literary form uses made up characters and plots has led to a use of the word "fiction" to mean a lie or untruth. Both forms of fiction, literary and lying, contain their own truths.
Literary fiction reveals the truth of the human condition, the drama of life, and often deeper truths than "non-fiction" genres. Motifs, symbols, characters, descriptive language, and plot all blend into words that may not represent "the world" as we know it. Yet, at its best, literary fiction gives its readers glimpses into truths about our world. So it is with many religious texts. Whether or not the words are literally true, at the surface level, religious texts, at their best, give readers (believers and non-believers) glimpses into truths about the human condition and the deep human drive to understand.
Believe it or not, lies often reveal truths, as well. First, there is the common adage that the best liars use seeds of truth in their lies. Second, what someone lies about often reveals what that person wants to hide more than it conceals it. Children, who have not mastered lying, often demonstrate this readily: "I didn't eat the pudding that was behind the milk in the refrigerator." Yes, you did.
Similarly, religious lies often reveal as much about the speaker of them as they conceal about "truth." Recently, a fanatically religious member of the Israeli Knesset (parliament) got up in front of the Knesset and decried a recent series of low-level earthquakes in Israel. In pornographically graphic detail he went on about how the acts of homosexuals and adulterers brought this punishment on us from G-d. Most less fanatical believers and probably all non-believers would recognize his words as religious lies. However, his graphic concern about what these homosexuals and adulterers did with their body parts shed a too bright light on his thoughts.
It is not the surface meaning of texts, literary or religious, or beliefs, that yield truth. Religion, at its core, provides systems for understanding our world and living with each other. These systems can be understood at deeper levels, and they represent an accumulation of human attempts to understand the world we live in.
The Jewish mystics speak about understanding religious texts and religion on several levels: surface, resonances, stories and explanations about the religion, and through all of these, glimpses of a deeper meaning to our lives. Gnostics, Buddhists, Christian mystics, Sufis, plus many more groups that have delved into religious meaning, all have similar understandings of religious texts and religion.
Ken Wilber has researched the mystical traditions of the major world religions, as well as fields such as psychology, physics, and philosophy. He finds that these areas, at their hearts, have more in common than they have differences. (See his A Brief History of Everything, for example.) He suggests that social science, science, philosophy, mysticism, art, and literature provide different perspectives on the same truths. He actually goes further, suggesting that at their core, their "truths" are more similar than different, despite their different perspectives, methods, and languages for speaking of "truth."
Truth is often found in fiction and fiction in truth. Why do we worry so much about the "truth" of religion? Religion holds truths that don't depend on facts. Facts often don't reveal truth in and of themselves without interpretation. All the world is subjective, given that we can only know it through our own analysis and interpretation. There are many systems for learning about the world around us and for learning about each other. The more we combine these systems to discover the nature of their core "truths," and the similarities that suggest "truth," the better.
Religion: truth or fiction? Yes, both. So what?