Any of the real world faiths that are represented in the game
Real world religions have been part of the game in all its incarnations. The original first edition Deities & Demigods described some 17 pantheons, of which five were fictional and a full dozen were in-game representations of real-world religions and mythoi. In 5th Edition, Appendix B of the Player's Handbook has a section on the Celtic, Greek, Egyptian, and Norse panthea, including alignments and suggested cleric domains for numerous gods.
Rather than choosing a fictional god that is nevertheless well-documented, you do have the option of choosing an in-game version of an actual historical deity. Doing so would give you access to a wealth of information and perspectives from completely outside the gaming community. If you want to play a character for whom religion is a big part of their life, you can access accounts of real people for whom the religion actually was or is a big part of their lives. The sources of information can include historical and anthropological accounts of the religion itself, psychological studies of what concepts in the religion represent (such as the work of Ginette Paris, who analyzes the Jungian archetypes present in Greek gods), and contemporary accounts of actual worshippers of reconstructed religions (here Neopagan adherents of the Norse pantheon seem particularly numerous). For example, the 5th edition Forgotten Realms deity Tyr is based on the actual Norse god Týr. In your research on how to play a cleric of Tyr, you would have access to accounts of the complex and nuanced worldviews of both historical and contemporary people who actually are adherents of Tyr.
How this has worked for me
One of my first players was, in his personal life, a Wiccan and practicing Neopagan. One of his characters was a cleric of Brigid, who he saw as an in-game version of the actual goddess Brigid. Although he was not in particular a devotee of Brigid, he did honor and celebrate the Triple Goddess, and his own research and experience were used in asking, what would this cleric of this goddess be like in a world where monsters were real and religion and magic were tangible? The goals and motivations of the character were informed by his understanding of the actual religion.
One of my first 5e characters was a monk who did not believe that magic was real. I could have tried to imagine an entire fictional monastic order around this central conceit. Instead, I decided to play him as Buddhist-in-outlook-but-not-name, with a belief that the magic in the game was just a particularly egregious example of how the world of the senses was maya (illusion) and was a mental block to his attaining enlightenment. My actual understanding of real-world Buddhism was considerably more shallow than my player's understanding of neopaganism, but it did provide me with a ready perspective for many circumstances, and a steady supply of stories and sayings and thus a richer role-playing experience.
As in any endeavor in which you are imitating the sincere beliefs of others, there is the potential for cultural appropriation and even sacrilege. Try to be mindful of this. Ask the other participants in your group how they feel about you playing a character of a real-world faith, keep your play in a private (not public) setting, and be up front about your intentions when requesting information from people. For example, if you are going to borrow a book from or interview a friend about their participation in a religion, let them know that it is for the purposes of playing a character in a game. If this use would offend them, respectfully take your search for information elsewhere.