As Novgorod it's usually best to use a Fabcricated Claim casus beli rather than a holy war. You should be able to raise enough men to take on any one pagan nation, but you'll have difficulties against 2 or more. In a holy war, it's very easy for pagans to team up against you. In a war over a fabricated claim they can only call in their actual allies. You can fabricate claims by sending your chancellor to the province you want and waiting until he succeeds.