Let's say all humans before salvation are blocks of wood:
Blocks of wood come in all sizes and shapes and colors. Some are solid. Some are rotten. Some are beautiful. Some are painted. But the one thing that they all have in common is that they all are made of wood.
The ball pit is filled with joy and love and childlike wonder. Blocks of wood don't work in this place. If we would put one in there it would sink to the bottom and be smothered (here is where the illustration has faults because we could throw a block of wood in there but when it comes to heaven we can't throw an unsaved soul in heaven -- but the point is that it wouldn't fit there or be happy there or even be able to live there)
When a block of wood wants to become a ball, it just needs to go to the creator and tell him he wants to be changed. The creator places that block of wood into his hand and transforms it into a ball. The transformed block of wood now can enter the ball pit and enjoy life and be surrounded by love and joy and childlike wonder!
Do you want the "magic" formula? See below:
"if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you shall be saved;" Romans 10:9