"The End" revealed that the "flash-sideways" reality was a sort of afterlife for the characters, but it was not heaven as popularly conceived in the West: most notably, although in general things were better in that reality than in their previous lives, they were not perfect. The second reality was a transitional state for the characters, who needed to "let go" before they could move on to yet another state of reality. This seems to echo aspects of Buddhist cosmology, in which a human who has practiced good karma — that is, a person who has followed dharma — may be reborn in a higher plane of existence, but those higher planes are themselves part of the cycle of samsara and therefore prone to suffering. In Buddhism, the only escape from this cycle of rebirth is to "let go" — of pain, of desire, of the illusion of self — and the ultimate letting go is enlightenment, by which one achieves nirvana.
But the "flash-sideways" reality is not exclusively a Buddhist parable. In the chapel where Jack meets his father, we see symbols of all major religions on the altar and in the stained glass window: the star and crescent of Islam, the Star of David (Judaism), the Aum (widely used as a symbol of Hinduism, but also present in Buddhism and Jainism), the Christian cross, the Dharmacakra (Buddhism) and the Yin/Yang (Taoism). Although reincarnation is not a widespread belief in the Abrahamic religions, the story told in "The End" resonates with one particular Christian parable: The Last Battle by C. S. Lewis.
The Last Battle is the final volume in the Chronicles of Narnia. In it, Narnia is destroyed. However, much of the action takes place in "Aslan's Country", which had previously been described as a sort of paradise. Here, Aslan's Country is revealed to be a copy of Narnia — or, rather, Narnia is a copy of Aslan's country, for everything in Aslan's country is more real, more perfect and more beautiful than its shadowy copy in Narnia. As one character explains:
"[The Narnia you knew] was not the real Narnia. That had a beginning and an end. It was only a shadow or a copy of the real Narnia, which has always been here and always will be here: just as our own world, England and all, is only a shadow or copy of something in Aslan's real world. You need not mourn over Narnia, Lucy. All of the old Narnia that mattered, all the dear creatures, have been drawn into the real Narnia through the Door. And of course it is different; as different as a real thing is from a shadow or as waking life is from a dream."
In The Last Battle, the characters in Aslan's country are repeatedly summoned to go "further up and further in". When they do so (climbing up through a waterfall on their way, just like Jack emerging from the pool at the heart of the island), they reach a garden in which they find "everyone you had ever heard of (if you knew the history of these countries)": all the characters that the readers of the Narnia books had grown to love, including many who had died. And that garden turns out to be yet another layer of reality, more real and more beautiful than the one before ("like an onion: except that as you continue to go in and in, each circle is larger than the last"). Finally, Aslan appears and reveals that the human characters (who had arrived in Aslan's country after a railway accident in England) are "as you used to call it in the Shadow-Lands, dead. ... The dream is ended: this is the morning."
"The End" doesn't map perfectly onto The Last Battle, but there are strong resonances. Like the human friends of Narnia, Jack and his friends realize that they are actually dead. Then Christian, acting as a psychopomp, summons them all through the Door, where they will find out where they're going next. (In the garden in Aslan's country, the friends of Narnia are greeted by the mouse Reepicheep, who had traveled to Aslan's country in a previous volume.) And it's surely no coincidence that the final gathering place is Eloise's church, above the Lamp Post station (the Lamp Post, of course, being a landmark in Narnia that marked a passage from one world to another). The Narnian and Buddhist parallels are both different ways of seeing the Lost finale: either way, the characters are moving "further up and further in", onwards to a greater reality, more beautiful and more true than the one before.
"It's all in Plato, all in Plato. Bless me, what do they teach them in these schools?"