At your table. It's an extremely reasonable take to believe it's either a god or someone sitting at the right hand of a god, like a saint, which makes the difference between a celestial warlock and the divinely powered classes mostly a matter of different mechanics.
Despite the screaming of warlock players on ENWorld, who either have been or are anticipating being traumatized by an aggressive DM, this is an extremely common take by anyone who hasn't yet had a warlock fan screaming into their face about the issue.
Warlocks are still in their 1E paladin era, where arguing about the class' flavor and who has a say over it are very much still open questions. It's definitely a class one should discuss with their DM before play, to make sure everyone's on the same page.