For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
Perhaps the most famous verse in the whole Bible. Hard to believe that God loved the world that much. Hard to believe that he gave his one and only Son. Hard to believe that anyone who believed. Hard to believe in eternal life. Yet it is true.
This is what the Gospel is about. If you reject this you reject everything. You may as well be a worm. The corrolary is that those who do not believe in him, shall perish.