Almost everything recorded about pre-colonial Tagalog religion was written by the people suppressing it, within a century of their arrival, and there is no indigenous written account to set beside it.
The Boxer Codex, an illustrated manuscript from around 1590, is the earliest substantial description, and it is exceptional for its coloured illustrations of Philippine peoples. Juan de Plasencia's account of Tagalog customs and religion, written for administrative purposes in the 1580s, is the single most quoted source for the deities and the offices, and Pedro Chirino added another early description. These are the foundation of the entire subject.
They were written to enable conversion and colonial government, they describe practices their authors regarded as devil worship, and they are the work of men who did not stay long and who worked through interpreters. They are also, in places, careful, and they preserve names and functions that would otherwise be entirely lost.
The pre-colonial script, baybayin, was in use at contact, but it was written on perishable materials and no pre-colonial religious text survives in it.
Later folklore collection, from the nineteenth century onward, gathered material that had continued under Catholic cover, and it is not straightforward to distinguish survival from later reconstruction within it.
Modern revival draws on the same colonial documents any scholar would use, which is worth stating plainly.