Despite the lofty title, Samuel D. Hunter’s A Case for the Existence of God is a play that at first might seem small, its subject matter as constrained as the little box of an office in which all the action takes place, dwarfed by the expanse of the Irene Diamond stage at Signature Theatre (scenic design by Arnulfo Maldonado). But as this sad and tender piece unfolds, it’s able to touch on universal questions by looking closely at the intersection of two ordinary lives during moments of particular vulnerability.