I think you are looking at this all wrong. Russia doesn’t care about ISIS and they are only fighting a portion of ISIS. Russia just wants Assad’s state to continue, because their ally Iran demands it, and because they want to use that port. But this isn’t a “Christians helping other Christians against Muslims” story, because Russia is there at Iran’s behest.
ISIS is the proxy in this. Now SA is threatening to put boots on the ground in Syria as well. The west is siding with the Sunni factions for now, represented in some part by ISIS, while Russia is with the Shias (Iran and the Syrian government). Russia cares about ISIS in that it represents those opposed to its own several interests - however hidden.