Lyric discussion by jf998247 

Cover art for Last Man on Earth lyrics by Mountain Goats, The

John has always had a really serious love affair with using Christian imagery from the Bible as literary lyrical metaphors for more real and mundane issues central to the human experience, i.e - love, hate, etc.

In this case, he's making a double entendre: a common statement of utter and total romantic disinterest is "not if you were the last man on earth". So, of course, John ties this in to Jesus, his betrayal, and his eventual return on Judgment Day as Messiah.

The result is something of the "don't count me out" trope expressed in a dualistic Christian context: no matter how long it takes for the day to come, or how many obstacles he must surmount, the narrator fantasizes about that "day of reckoning" when all the liars and falsehoods are stripped away, on which he will be able to say "Now that the liars are all gone, I tell you that I never denied you. I never left you. And I have always loved you" and that will simply be enough, when it was never that simple enough before.

John's also a smart dude, so I suspect he realizes that the whole thing is a sad, unlikely fantasy a la BNL's "Some Fantastic" or Henley's "Boys of Summer". What connotations that has for the zeal of John's belief in Christian doctrine remains to be seen.