To be dying; to be approaching or very close to death (i.e., about to be admitted into the afterlife). My grandfather fought a good fight against his stomach cancer, but I'm afraid he's knocking on heaven's door now.

Dying; approaching or very close to death (i.e., about to be admitted into the afterlife). My grandfather fought a good fight against his cancer, but I'm afraid he's knocking on heaven's door now.