Every written source for pre-Christian Slavic religion was produced by Christians describing what they were in the process of destroying, and most of them were foreigners as well.
The Rus Primary Chronicle records Vladimir's idols and then their destruction. Helmold of Bosau and the Chronicle of Thietmar of Merseburg describe West Slavic sanctuaries from the position of the German church. Saxo Grammaticus gives the fullest single account, of the temple of Svetovit at Arkona on Rugen, and he wrote it as a Dane celebrating the campaign that levelled it in 1168. Procopius, writing in the sixth century, offers the earliest notice and the least detail.
None of these authors was interested in Slavic theology except as error. They record idols, temples, tribute and oracle, because those were the institutions that had to be dismantled, and they say almost nothing about what anyone believed.
The second body of evidence is folklore collected in the eighteenth and nineteenth centuries, immensely rich and a thousand years downstream of conversion. Some of it plainly preserves older material under Christian names, the transfer of Perun's attributes to the prophet Elijah being the standard example. Reconstructing an original from it is a method with a poor record, and the Romantic era produced a considerable literature of invented Slavic gods that still circulates.
Archaeology supplies the third leg, and it is the one most likely to move: sanctuary sites, idols, and the debated Zbruch figure.