If you have a belief in God, you must have a belief in eternal life, because the Gods of all religions declare it.
All the religions of the world say that life is eternal.
The afterlife played an important role in Ancient Egyptian religion, and its belief system is one of the earliest known. When the body died, parts of its soul known as ka (body double) and the ba (personality) would go to the Kingdom of the Dead.
Writing that would later be incorporated into the Judaism and the Old Testament names sheol as the afterlife, a non-descriptive place where all are destined to go after death. The Book of Numbers refers to people going down to sheol when the earth opens up and destroys the rebellious Korah, Dathan and Abiram and their 250 followers (Numbers 16:31-33). One might take this as implying that sheol is literally underground, although it is as easily read literally, as signifying an earthquake or split in the earth.
The Talmud offers a number of thoughts relating to the afterlife. After death, the soul is brought for judgement. Those who have lead pristine lives enter immediately into the "World to Come."
Christianity ~ When questioned by the Sadducees about the resurrection, Jesus made it clear that the resurrected will be like angels in heaven. The Revelation of John tells that the blessed will enjoy the vision of God for eternity.
The Islamic belief in the afterlife as stated in the Qur'an is unique, its official description is more detailed. The Islamic word used to describe Paradise is jannat - this is eternal life.
The Bhagavad Gita, an important book for Hinduism talks extensively about the afterlife. Here, the Lord Krishna says that just as a man discards his old clothes and wears new ones; similarly the soul discards the old body and takes on a new one. In Hinduism, the belief is that the body is but a shell, the soul inside is immutable and indestructible and takes on different lives in a cycle of birth and death. The end of this cycle is Moksha or salvation. This is eternal.
Buddhism, Sikhism and Jainism all teach reincarnation until the balance of karma is finished, and moksha, release from the cycle of birth and rebirth is ended. Moksha is eternity in the presence of God.
Zoroastrianism states that the urvan, which is the disembodied spirit lingers on earth for three days before departing downward to a kingdom of the dead, which is ruled by Yima.