I think holy knight hero is really tired to be a saint everywhere he go, so he try to escape this by going dark edgy knight 8th syndrome and go away from his country ; but he's hopeless with anything other than light magic (where he's OP as fuck) and has the godess from this world having personnal attention to him since he's her personnally chosen holy knight.
Well, it's predictable from the first chapter
it will be comedy/adventures (and not isekai !!)