‘An incredulous thing, to make you incredulous.’ Which word is wrong?
Neither, as each usage to that meaning can belong;
‘An incredible thing to make you incredible’ — can this also go?
No, for here ‘Incredulous’ the second one should show.
Annoyingly, people are tending to revive the meaning of ‘incredulous’ to include ‘incredible’, as was the case a couple of centuries ago. Shakespeare used it as such.
The distinction has worked well for two hundred years, though, but now some people who are either ignorant or trying to be smarty-pants are bringing it back to mean ‘unbelievable’ rather than ‘unbelieving’. I would definitely edit it out if it appeared in a manuscript I was going through. ‘Incredulous’ in the recent understood sense of ‘unbelieving in an amazed way’ is too valuable to have it polluted with an additional meaning for which a perfectly good word already exists.