An agnostic is someone who believes that there can be no proof of the existence of God, but does not deny a possibility that God exists.
15 seconds sound clip from the Donnie Darko movie soundboard.
You can hear this line at 01:44:02 in the Blu-ray version of the movie.
Quote context
[...]
- The sky is going to open up.
- If this world were to end, there would only be you and him and no one else.
- You can stop taking your medication. They're placebos, just pills made out of water.
- An atheist is someone who denies altogether the existence of God. You're an agnostic.
- Hey. I got in. I'm going to Harvard.
- Okay, but it has to be small, all right?
- We got eggs, water balloons and a dozen rolls of toilet paper.
[...]