If Roman legend is to believed, Constantine received a vision of the cross and the words "by this sign you will conquer" prior to the battle of Milvan Bridge inspiring his conversion to Christianity. While this is probably propaganda, this would definitely count as ASB if true.
Regardless, the speed at which at which Christianity went from being a fringe occasionally persecuted off-shoot of Judaism to the central pillar of Roman, and then European, society would called ASB if it appeared in a fictional timeline.