Etruscan religion is read almost entirely at one remove, and the reason is a shortage of Etruscan writing about it. The alphabet is readable, since it is a version of the Greek one, so the sounds of the language are known; the language itself is not related to Latin or Greek or to any other well documented tongue, and the surviving corpus is dominated by short funerary inscriptions giving names and offices. Long texts are very rare.
The longest of them is a linen book that survives because it was cut into strips and used to wrap an Egyptian mummy, acquired in the nineteenth century and now in Zagreb. It is a ritual calendar: dates, gods, and the offerings due to them, running to some twelve hundred readable words. It is not a narrative and it explains nothing, but it is the closest thing there is to an Etruscan liturgical document written by Etruscans.
Everything else comes through Roman and Greek authors, and they are interested in the parts that are useful to them. Cicero writes about divination as a Roman magistrate arguing about whether it works; Livy records prodigies and their expiation because they mattered to the senate; the later antiquarians preserve fragments of the discipline in the course of explaining Roman practice. The gods are visible in Etruscan art, on mirrors and tomb paintings, often labelled, which is why the pantheon is better known than the theology.