I'm willing to convert to Orthodoxy. But Lithuanians are Catholic too.
But as I would imagine, and the need to fill up Orthodox "voids" in the East, all fomer Warsaw pact nations should (most are allready) but most should go to Orthodoxy.
First a common people (slav) would have to exist first. I'll put a check mark next to that.
Than a common "movement". No need for a common language, that would be impossible, but a purpose, one, and some unifying force such as religion. I'll put a check near common purpose but not yet so a common religion. The common religion will act as a brotherly bond, one unbreakable along with slavdom.
Three, recruit nearby allies, and rest assured.