How do you ripen a persimmon after it has been picked?!
Answers: It is halfway orange now, but it need to ripen all the way. I have eaten under-ripe persimmon and...blech!! Tastes like sucking on a mouthful of sour flour!
Ethylene gas will help ripen your persimmon. Since ripe bananas give off a lot of ethylene gas (which is what gives bananas their signature smell), you can put your persimmon in a paper bag with a ripe banana for a few days (check on it every day, take it out when it's ripe enough to suit your tastes).
Some other ripening methods I found are putting it in a paper back with a ripe apple, and freezing it for 24 hours, then defrosting.