If most of us wind up going to heaven, wouldn't it be too crowded? What if there's no room? I just can't see how God can handle all of us! I really hope you answer me; I think about this all the time. — From M
I also think about the "world to come" (the Jewish term for heaven) though knowing what happens there or even whether there is a heaven is way above my pay grade. The belief of the Abrahamic faiths is clear: Death is not the end of us. This depends upon a belief that we are made up of bodies and souls. Our bodies are material and take up space, but our souls are immaterial and take up no space. This makes your overpopulation problem go away. All the souls just kind of fit together in one immaterial place that takes up no space. Gravity is everywhere and acts on everyone but takes up no space. Heaven is like moral gravity.
There is, however, another spiritual solution to overcrowding in heaven: reincarnation, the belief that our souls after death and judgment are reinserted into newborn babies. Hinduism, Sikhism and Jainism have embraced this as one of the central tenets of their faith. Buddhism rejected the Hindu belief in the soul (anatman) and so rejected reincarnation. Some parts of Buddhism do embrace reincarnation; Tibetan Buddhists consider the present Dalai Lama to be the 14th reincarnation of the original Buddha.
Reincarnation was not developed by Hinduism to solve a housing shortage in heaven. It evolved as a spiritual corollary to the doctrine of karma, the belief that every single one of our acts — good and evil — imprints itself on our souls. If the good outweighs the bad at the end of life, our souls are re-born into a higher state of existence. The opposite is true if the evil outweighs the good. We could be reborn as a king or a cockroach; Jains sweep the pavement where they walk to avoid inadvertently stepping on Uncle Murray. Release from this cycle of rebirth (called moksha) is the ultimate spiritual goal for the eastern faiths.
I don't know if I believe in reincarnation because I do not spend much time thinking about what happens after death. I try to do the good because it is the good — not for reward (or as a cockroach-protection policy for my next life). The idea, however, that God's love would give us another chance to get things right in life seems a lovely way to imagine God's love.
The idea of reincarnation is not generally supported by Judaism, but certain Jewish mystics who follow Kabbalah believe in it, calling it gilgul ha'neshamot in Hebrew.
Reincarnation is basically absent from Christianity, in which the fundamental belief is that we live once and are judged forever: "And just as it is appointed for man to die once, and after that comes judgment." (Hebrews 9:27)
Christians attracted to reincarnation pin their hopes on Matthew 17:10-12: "And his disciples asked him, saying, Why then say the scribes that Elias [Elijah] must first come? And Jesus answered and said unto them, Elias truly shall first come, and restore all things. But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them."
This passage seems to imply that John the Baptist was the reincarnated soul of the Hebrew prophet Elijah. Nevertheless, the text in Matthew could simply be referring to the belief that John completed the role Elijah was meant to fulfill, announcing the coming of the messiah. Unfortunately, those trying to make a case for Christian reincarnation based on Matthew have to cope with John 1:21, where John the Baptist is explicitly asked if he is Elijah and he answers, "No."
I have great admiration for the eastern belief in reincarnation, but I believe that we ought to try our best not to act like a cockroach in this life, even if we are not reborn as a cockroach in the next life.