There is very little to reconstruct from. No pre-Christian Slavic text exists. The chronicles list some gods without explaining them, the western Slavic temples are described only by hostile foreign clerics, and the archaeology is sparse and often ambiguous; even the Zbruch idol, the best known Slavic cult sculpture, has had its authenticity questioned.
That gap has been filled twice over. The most consequential filling is the Book of Veles, a text presented as pre-Christian Slavic scripture carved on wooden boards, which surfaced among Russian emigres in the 1950s on the strength of a claimed discovery in 1919. Philologists examined it and found its language incoherent as any historical stage of Slavic, mixing forms from different centuries and different languages. Academic Slavic studies regards it as a twentieth-century forgery, a judgement argued in detail by Lidia Zhukovskaya in 1960 and not seriously contested since. A number of Rodnover groups nevertheless treat it as sacred text, and others reject it outright; the disagreement runs right through the movement.
The second filling is folklore. Rodnovery draws heavily on nineteenth and twentieth-century collections of peasant custom, on the assumption that Christian practice in Slavic villages preserved a pagan layer beneath it. That model, often called double faith, is itself contested: scholars including Eve Levin have argued that it projects a division onto what participants experienced as a single religion. Some continuity is certainly real, as the seasonal festivals show. How much is unresolved, and on the surviving evidence probably unresolvable.