The goodness of anything we’d be willing to die for is something about which we have a degree of certitude (i.e. assent to some claim or fixity in it) equal to scientific certitude. Scientific or at least putatively scientific movements (Marxism) can create this level of devotion, but only in an impersonal sense since science as such can only be had about what is abstract. What one dies for when he dies for a scientific movement is not this country or party, but for this and for anything else that would be relevantly similar. But saying “I pledge my life to you and to whoever or whatever else might be relevantly similar” is not something one can say out of love (try the line on your spouse.)
And so if to be willing to die out of love is possible, we need a certitude equal to the highest form of scientific certitude while nevertheless not being scientific. Said another way, we require there to be some rational way to give a degree of assent equal to what we give to the self evident, while it is nonetheless not given to something self evident or anything that follows from it. This is the sort of object we have in mind when we make an oath of fidelity or (if we replace the abstract noun with a concrete one) when we have faith in something.