If God didn't exist (as your uncle claims), then we wouldn't have any reason to believe in life after death. Nor would we have a soul or spirit that could survive beyond the grave, and death would indeed be the end.
But God does exist–and that makes all the difference! God made us, and He put within each of us something of Himself–which we call our spirit or soul. Someday our bodies will die, but our souls will live on–and eventually, the Bible says, God will give us new bodies that will never die. The Bible says, "Now we know that if the earthly tent (or body) we live in is destroyed, we have a building from God, an eternal house in heaven, not built by human hands" (2 Corinthians 5:1).
How do we know this is true? I could list several reasons–but the most important is because Jesus Christ died and then came back to life again. By His resurrection He proved beyond doubt that this life is not all there is, but ahead of us is eternity. His promise is true: "I am the resurrection and the life. He who believes in me will live, even though he dies" (John 11:25).
But Jesus didn't come just to prove to us that there is life after death. He came to open heaven's door for us–and He did this by removing the only thing that can keep us out of heaven: our sin. Open your heart to Jesus Christ and discover the hope He alone can give you–the hope of life in heaven with God forever.