There really aren't written sources about Finland before Christianity arrived here, as the current day area of Finland became part of Sweden then. There are only two rune stones that mention about Finns, but even then they don't say much and we are not even sure if it's about Finns or Sami or some other people. Then some other earliest more proper sources are from 12-13th century, from a time that crusades where happening here and are kind of after Christianity already had started arriving..

There honestly aren't much sources about pre-Christian Finland, other than say archeology etc.