Jesus says in John 8:24, "I told you that you would die in your sins; if you do not believe that I am the one I claim to be, you will indeed die in your sins." Obviously all people die, so the death Jesus talks about is referring to separation from God by being justly punished for our sins eternally in hell.
In 1 Thessalonians 4:14 we read, "We believe that Jesus died and rose again and so we believe that God will bring with Jesus those who have fallen asleep [died] in him."
What does this mean??
Well, God is loving, holy, and just. Justice requires punishment of crimes... we all know this in our court systems today. Since God is infinite, His justice far surpasses our human standards of justice. To remain holy and just, God must punish sin. The greek word for sin is an archery term that means "to miss the mark," meaning the exact bulls eye was not hit. We all have sin, in that we fall short of God's required perfection. To let sin in His presence would corrupt His justice, sin must be punished. So either we spend eternity in hell... or option 2.
What's option 2 you ask?
2000 years ago, God Himself (in Jesus Christ) stepped into the courtroom of humanity, and died in the place of everyone who chooses to believe in Him... Then He was resurrected the 3rd day to prove that we too will have eternal life in Him. This was the ultimate expression of God's love for us. If we believe we are sinners (missing the mark of perfection), and ask for forgiveness (turning from our sin, and toward God), and believe that He died for us and took the punishment we deserve in our place, and that He rose again... then we are saved from facing the punishment of hell ourselves. This doesn't mean we're perfect, but that we strive for holiness by seeking after God from then on.
Good people don't get into heaven, forgiven people do... those who have accepted and believe in the payment Jesus made on their behalf.
