<p>Those are valid questions. Sorry I didn't explain more. As a Christian, I grew up with all the stories and doctrine, and I tend to think everyone else knows it as well, even if they don't believe in it.</p>
<p>Anyway, Christians believe that God created humans to be perfect. Adam and Eve were thus originally perfect, without sin. This means they were in really close communion with God, so in a sense it was literally heaven on earth. But then they ate the fruit that God had expressly forbidden them to eat, and sin entered the world. Every human thereafter has sinned (Romans 3:23). Because of that sin, all the imperfect things in this world (suffering, death, poverty, pain, etc.) came as a consequence. Some ask why God didn't create humans so they couldn't turn against him and sin. But that's the whole concept of free will. Basically, if God made it so that we had to love him (and would thus all go to heaven), we would be nothing more than robots. How much does it mean if someone loves you, when they are basically forced to do so?</p>
<p>To Christians, the punishment for sin is death (both literally, and figuratively - meaning Hell). However, if an innocent is sacrificed, he can take the punishment of that sin (like a scapegoat). That was the principal used for animal sacrifices in biblical times, which is how people before Jesus' time could go to heaven, as long as they had faith in God. However, they were still separated from God, as they had to repeatedly offer up sacrifices to stay "pure".</p>
<p>But God loved the world, and didn't want this separation between himself and us. Not that we deserve this love, as sin is basically turning your back on God and his commands. But "God demonstrated His love for us in this: while we were yet sinners, Christ died for us." (Romans 5:8) Christ Jesus, God's son, abolished the need for animal sacrifices. (Thus why he's often referred to as the sacrificial lamb in the Bible.) Christians believe that He is perfect, and so when he died, his sacrifice took the punishment of all the sin in the world. It's like what Aslan did for Edmund in the Chronicles of Narnia (pardon the reference, I just watched the movie), only Christ took the punishment for everyone in the world as the Ultimate Sacrifice. </p>
<p>Thus, though we don't deserve it, we are given salvation and eternal life (meaning we go to heaven). All that's necessary is that we as humans receive that gift. In other words, we must believe in what Christ did (Romans 10:9-10). Christians believe in that, and therefore Christ's sacrifice atones for their sins and they are seen as perfect in God's eyes. Non-Christians do not believe that, and thus are still imperfect and unable to go to Heaven.</p>
<p>I hope that made sense, and wasn't too long...</p>
<p>Oh, and out of curiosity, what do you mean by "what happened in the Garden of Gethsemene"?</p>