Once upon a time, God made the Earth in six days. On the sixth day, God created the first man (Adam) and told Adam that Adam could eat the fruit of any tree except the fruit of the Tree of the Knowledge of Good and Evil. Then, because Adam was lonely, God created the first woman from one of Adam’s ribs. Her name was Eve. Unfortunately, a talking snake tricked Eve into eating the forbidden fruit, and then Eve tricked Adam into doing the same. God punished Adam and Eve and all their descendants by forcing them to live in a harsh world. Their lives would be full of suffering and end in death.
But God still loved Adam, Eve, and the rest of humanity, so God decided to save some of them. To understand how God will save some people, you have to understand that God is three persons who share one essence or substance. Thus, God is not only one; God is three. And God is not only three; God is one. God is the Trinity: the Father, Son, and Holy Spirit.
You also have to understand that death is not the end of a human’s conscious existence. God made Heaven and Hell so that humans will live forever, whether they want to or not. Because the first two humans disobeyed God, God (who is love) decided that all humans deserve to burn alive in Hell forever after they die. But because God loves all His creatures, God decided that He will save some humans from Hell and get them into Heaven. And this is how God will do that saving.
The Holy Spirit impregnated a virgin girl named Mary with the Only-Begotten Son of God. The result was a baby who was both fully God and fully human, named Jesus. Jesus united God and humanity in Himself, thus ending the separation between God and humanity caused by the original sin of Eve and Adam. Not only that, but Jesus lived a perfectly righteous life, never sinning and always doing what was good. This perfectly righteous life ended with a perfectly righteous death, which somehow atones for the sins of all humanity. A sin is an action that breaks a commandment of God, and any sin requires a perfectly righteous sacrificial death to make it not a Hell-worthy offense. Someone must pay a debt so that the debtor will not be punished. Humanity owed the debt of Hell to God for disobeying God’s commands, so God paid that debt to Himself by having Himself murdered.
You might be confused here. If Jesus’s incarnation, life, and death atoned for all of humanity, why will many people still burn alive in Hell? The answer is that people need to be baptized, believe in this nonsensical story, eat Communion, and live a very good life. It’s like having one million dollars and being hungry. To get food, you have to spend some of that million dollars. Likewise, to go to Heaven after you die, you have to get a magical bath, believe the right nonsensical story, eat some magical bread and wine, and generally be a good person as defined by God’s holy organization (the one, true Church) and/or God’s holy book (the Bible).
Jesus will come again soon, although Christians have been saying that for approximately 2,000 years. After Jesus returns, He will fix the entire universe so that it will be a nice place to live again, rather than a harsh one. Jesus will also resurrect the dead and give all people immortal bodies. Some of these immortal people will have uninterrupted happiness with God in Heaven forever, and the rest will have uninterrupted misery without God in Hell forever. So some people will live happily ever after. The end.