In Christian theology, Heaven is what brings God's plan of humankind's redemption to completion. In the beginning, God created Adam and Eve in his image, and they were like him – perfect, without sin. But they ate from the forbidden tree, rebelling against God, and God cast them out of Eden and condemned mankind to the sinful desires of the flesh. But God loved his creation, and wanted to redeem them. He created a system of ritual animal sacrifices, in which a high priest killed an animal (usually a lamb) in a holy temple, and the blood of the animal was atonement for humanity's sins. But the covenant was imperfect, because an animal had no capacity for good or evil, and no human was without sin. So God promised a messiah that would bring a new covenant. And sure enough, the messiah came. The messiah was God himself in human form. He died on a cross, acting as both the high priest and as the perfect lamb. And while a high priest was required to remain standing to show his work was never done, Jesus rose from the dead and sat at the right hand of God, indicating his work was finished. Now people could be saved simply by believing in Him. Lambs everywhere breathed a sigh of relief. But that's not the end. God will come back, destroy the world and send sinners to Hell. He'll create a new Earth and a new Heaven, where the saved souls will reside in peace forever, being returned to their sinless, perfect state – just as Adam and Eve were before the Fall.
In case you think I made any of that up, I simply suggest you read the book of Hebrews, and peruse the book of Revelation. It's all in there. Let's ignore some oddities for now, like...
- Why would God bother with an imperfect covenant? Why is the whole darn thing so complex at all?
- If the Messiah is God in human form, and he is the sacrificial "lamb", he's literally sacrificing himself to himself to pay a price he mandated to free us from a curse he put on us.
- What's to keep humans in the new Heaven from rebelling?