Instant timing still requires priority. But when Valgavoth's Faithful resolves and enters the battlefield, unless some trigger goes of because of that, the stack is empty, it is its controller's main phase, and after any object on the stack resolves, the active player receives priority, in this case the Faithful's controller. That is sorcery timing right there, for your opponent. You do not get to cast an instant now, because you do not have priority. You opponent did not pass it, they used it to activate the Faithful.

By the way, that is the same reason, why you can't kill a planeswalker after it resolves and before one of its loylaty abilities is used. If the player uses one right away, you never had priority to do so.