According to Theravada Buddhism, what they can see as ghosts are called mental bodies, or in the Pali language "manomaya kaya" or "gandabbha". When a human dies, he many not necessarily be reborn immediately into another type of being, such as a heavenly or hellish being or animal, if his human existence (Pali: "bhava") is not over yet. When a human dies with his human bhava still not over, he would be in the gandabbha form until a suitable womb is available for him to be reborn into as a human again. The waiting period ranges from days to months to years to even centuries. Only humans and animals have this gandabbha form because they have to grow in a physical body from a small foetus into adulthood. Other types of beings are reborn spontaneously into a full form. Examples of beings that are spontaneously reborn are devas (sensual heavenly beings), brahmas (non-sensual heavenly beings), hungry ghosts (Pali: peta) and hell beings. If a human dies at the end of his human bhava, then he would be reborn as a different being altogether, such as a deva or as an animal. If reborn as an animal, there might be a waiting period as an animal gandabbha. Hungry ghosts are different from human gandabbhas because they have deformed bodies and they are spontaneously reborn and they suffer from extreme hunger, whereas human gandabbhas are just stuck in a waiting period.
Most Christians don't believe existence of ghosts. And only believes going to Haven after death...to them no Hell, no ghosts...They can even destroy their own Ancestral Tablet or buddha statue after converting to Christianity,they don't believe Ancestral worship at all...
