Let's say you are walking through the woods with some friends. You come to a clearing and there in front of you is a raging river.
Just when you think you will have to turn around and go back you spot a bridge about half a mile down the bank of the river. You all walk down to the bridge and examine it. Will it hold you? You believe it will hold you and so do others. They step onto the bridge and start walking across it. Seeing them you now think it will hold your weight, too. But are you really believing that bridge will hold you? You will only truly believe that the bridge is capable of holding you up when you are willing to take that step onto the bridge itself and trust it with your life.
It is the same way with Jesus. In your head you can believe Jesus exists, was a good teacher, did miracles, even died on the cross. But that is just knowing Him intellectually like some historical figure. Jesus wants you to know Him personally. Jesus died on the cross and paid for your sins in full. He became your 'bridge' to Heaven. You must trust Jesus completely as your way to Heaven just like stepping on that bridge and committing your life to it.