Am I religious if I say there isn’t a marble at the table? Or a walnut?
Yes.
A rational person would say “I don’t know if there is a ball, marble, or walnut”. If you have experience with other tables with upside-down cups, you might go further and say something like “If this table with cups is like the other tables with cups that I’ve experienced… (fill in whatever your experience re:balls, marbles, walnuts, etc)”. To say more requires a leap of faith.
And, unlike tables, cups, balls, and walnuts, the existence of the universe is apparently quite singular. Thus, if you haven’t had direct experience with it, it is unlikely that you’ll have had sufficient parallel experiences to make any meaningful statements like “If this universe is like the other universes I’ve characterized…”. Therefore, lacking any direct experience, the reasonable position would be “I don’t know”.