What happens if I choose not to discard the found card? Do I put it as on-deck card or play it?
On page 16 of the rules there is a detailed description for Cleansing Rain. Towards the end of the description there is this, "If you choose not to discard the card, you must still shuffle your deck."
So you can either choose a card and discard it or look through your deck and reshuffle it.