Every surviving description of Slavic religion was written by Christian outsiders recording what they intended to destroy, and none of them records a cosmogony. The dualist earth-diver story usually printed as the Slavic creation myth is attested only in folklore collected after a thousand years of Christianity, in a form shared with Finno-Ugric and Turkic neighbours. Attempts to recover a pre-Christian original from it have produced competing reconstructions and no agreement, and most specialists have stopped trying.
What would resolve this A pre-Christian Slavic text of any length would settle it, and none is known to exist. Short of that, an inscribed or iconographic find from a pagan-period sanctuary that depicts a creation sequence, of the kind Arkona or Zbrucz might still yield, would give the reconstruction something outside the folklore to stand on.
OpenSlavic philology and the archaeology of pagan-period sanctuaries in the western Slavic landsMircea Eliade (ed.); Lindsay Jones (ed., 2nd ed.), Encyclopedia of Religion