(revised from ”Re: response to victor & malaki)
It is interesting to ponder whether or not Southeast Asians had a concept of “god” before the advent of Hinduism. There are quite a few religions where Supreme Beings do not really exist, for example, Buddhism and Taoism, but in these days, we tend to get all riled up about the People of the Book (meaning, Zoroastrians, Jews, Christians, and Muslims)
But the discussion of “god” is hampered because of the vagueness of the term, which I think encompasses at least three different concepts: (1) a single benevolent creator entity found in monotheistic religions: e.g., God, YHWH, Allah, Ahura Mazda (2) mostly human entities with some superhuman powers: e.g., Zeus, Odin, Tiamat, Vishnu (3) spiritual forces associated with natural, domestic, or agricultural phenomenon: kami from Japan, genii from Ancient Rome, elves from Scandinavia.
From what I understand, spiritual beliefs in Southeast Asia were more resonant with concept (3) I’m also not sure that “worship” is necessarily the right term, or more likely, “worship” means different things depending on which god-concept you subscribe to, in which case it seems likely that the people who inhabited what later became the Philippines worshipped spirits that governed nature, home life, and agriculture. I recall that the spirit of rice was very prominent, as were the spirits of the jungle, the river, and the sea. I vaguely remember watching a video about how the forging of the kris blade was a ritual regarding the spirit of the sea, but I may just be making this up now.
Also, I think it is in the Darangen (the definitive epic poem of the Maranao people) that I first came across the word diwata used to refer to such spirits. The same word is found in Tagalog to mean, among other things, according to the dictionary, muse and goddess, and is undoubtedly related to the Tagalog word diwa, which I understand means soul or spirit.
Of note, diwa and diwata look obviously related to the Sanskrit word deva which mean deity, and through the putative Indo-European mother tongue is in turn related to Greek theos and Latin deus, meaning god.
In any case, it’s true that there aren’t any documents from the pre-Hindu era. Which makes sense since it seems generally believed that the writing systems evolved from Indic sources. But a lot of the information is apparently preserved in living culture, if my professor is to be believed. True, there aren’t really any “pure” Austronesian cultures anymore, but, much as Celtic animistic beliefs (i.e., the beliefs of the Druids) persisted in England despite the introduction and dominance of Roman, Scandinavian, and finally Christian beliefs, a prevalent idea is that a lot of the ancient Southeast Asian rituals are couched in terms of Hindu, Islamic, and Christian beliefs. (Like the anecdote that when Catholicism was introduced into the Philippines, saints simply took on the characteristics of whatever local spirits or deities were worshipped.) I immediately think of Maria Makiling, and the Santo Niño.
So, yeah, we will never know, but we can make educated guesses.