works equally well without a supernatural interpretation
The screenwriter has said that he meant Jacob's ascent into Heaven and reunion with his son to be literal.
However, just taking what's on screen at face value, the film works equally well without any religious or supernatural interpretation. If we take the entire course of events to be fantasies, memories, and hallucinations in the mind of a dying man, and his final "ascent" to be accepting his own death and being at peace with it (without any afterlife), the movie still makes sense.