And yet I still have a residual feeling for accusative here and do not know why. I wonder if I am thinking "I live, taken under divine protection" (as in to be brought sub imperium Romanum)? Or is it Virgil's visa sub obscurumnoctis? Sub auspicia seems to be later Latin. And then there is this hymn, Sub Tuum Praesidium (in which movement is suggested, flying to a source of protection). No matter...as a reference to a state of being, ablative seems the way to go.