Is Infernal a language during the war in heaven? That is, were the fallen angels speaking it by the last battle in the teaser? I wonder since the players may remember hearing it back in Heaven iwhen they hear it in the Mortal realm.
If so, how would a character go about learning the language? Or on that note how did Infernal come about if all angels spoke the same language at the start? You could rationalize that by having it that having an Infernal Taint are incapable of speaking the Divine Tongue much like witches were incapable of saying the Lord's Prayer.
What "religion" would a starting cleric (d20) be who was in the teaser and thus in heaven? What domains? It wouldn't make sense that he belonged to any of the church orders and the atavistic church doesn't seem appropriate either.
I would say you could pick any domain since clerical spells hadn't been separated yet - kind of like Arcane magic and the realms in RM. However I don't see why being in Heaven would necessarily eliminate church orders. They very well could have existed then - much like various monastic orders in the Catholic church that emphasize different things.