"I put my hand on the altar rail. 'What if ... what if Heaven..." — David Mitchell quote meaning

I put my hand on the altar rail. 'What if ... what if Heaven is real, but only in moments? Like a glass of water on a hot day when you're dying of thirst, or when someone's nice to you for no reason, or ...' Mam's pancakes with Toblerone sauce; Dad dashing up from the bar just to tell me, 'Sleep tight, don't let the bedbugs bite'; or Jacko and Sharon singing 'For She's A Squishy Marshmallow' instead of 'For She's A Jolly Good Fellow' every single birthday and wetting themselves even though it's not at all funny; and Brendan giving his old record player to me instead of one of his mates. 'S'pose Heaven's not like a painting that's just hanging there for ever, but more like ... Like the best song anyone ever wrote, but a song you only catch in snatches, while you're alive, from passing cars, or ... upstairs windows when you're lost ...