Dīn, meaning religion or belief in modern Persian and Arabic, has its roots in Zoroastrian cosmology.
In the Young Avesta, it was stated that after one’s death, an entity named Daena would lead the soul to be judged by Sroush. After Sroush weighed a scale with one’s good and bad deeds, the soul would have to cross a bridge called the cinvata peretu, meaning “the bridge that can be built up.” If the soul’s scale weighed heavily in favor of his good deeds, the bridge would be wide and sturdy, leading to Heaven. If not, the bridge would be a narrow and rickety, and the soul would fall to Hell.
Anyway, it is from Daena’s name that we get the word dīn today. It’s related to the Persian verb dīdan, which means “to see.”
I think it’s a really beautiful word, actually, because it implies that with faith, you can see.