Warning: Spoilers for Fantastic Beasts: The Crimes Of Grindelwald ahead.
Fantastic Beasts: The Crimes of Grindelwald is chock full of plot twists and big reveals, but none is bigger than the moment at the very end of the film when Grindelwald finally tells Credence who he really is: Dumbledore's secret brother, Aurelius. For most of the film, the viewer is being led to believe that Credence is Corvus Lestrange (Leta Lestrange's brother), but even after this is revealed to be untrue (thanks to Leta accidentally murdering her infant sibling), no one could have expected this particular twist.
Of course, the Credence/Aurelius reveal is one that has fans spinning, especially as the Dumbledore family history is fairly well explored in the original Harry Potter series and a brother nearly two decades younger than Albus is never so much as hinted at. Of course, it's not the only part of the film that retcons established Harry Potter canon (The Crimes Of Grindelwald plays it pretty fast and loose with Harry Potter lore throughout), but it's the biggest one - and that leaves us to wonder if Grindelwald (and J.K. Rowling) are being a little less than truthful about Credence's identity.
All that Grindelwald tells Credence is that he has a brother who "seeks to destroy him", and that his name is Aurelius Dumbledore. He backs up this assertion by telling Credence "There is a legend in your family that a phoenix will come to any member who is in dire need." and then, taking a chick that Credence has been caring for, he launches it into the air, where it catches fire and bursts forth as a phoenix. Dumbledore himself speaks in Fantastic Beasts 2 about the connection that his family has to the phoenix, which seems to suggest that Credence's bird proves that he is, indeed, a member of the Dumbledore clan.
However, there's a good chance that Grindelwald is lying. He's a manipulative Dark Wizard, and has a blood pact with Dumbledore meaning that he can't take down his biggest rival. There's little reason to doubt that he would happily lie to a powerful Obscurus in order to make him hate Dumbledore and attack him on Grindelwald's behalf (believing that he is doing it for himself, of course). It's also worth noting that Grindelwald spent most of the franchise seemingly uninterested in Credence himself, only wanting to foster a connection when he realized how powerful Credence could be - not the behavior of someone who knew his secret identity from the start and wanted to restore it.
There's also a simple way that Grindelwald could pass off his Dumbledore lie with the help of a spell that Hogwarts students start learning in their first year: Transfiguration. One of the most complex branches of magic, transfiguration isn't easy, but it is useful; and forms of transfiguration are used in many common spells. Grindelwald could easily have simply transfigured Credence's chick into a phoenix for a few moments - enough to convince the awe-struck boy (who was not raised in the wizarding world) of Grindelwald's lie.
Of course, there are plenty of rules around the use of magic that seem to only come up when they are useful, and it may be that it is impossible to transfigure something into a phoenix. At this point, we know that food is one of the Five Exceptions to Gamp's Law Of Elemental Transfiguration, and a phoenix could be one of the other unnamed four... but that seems like an extremely convenient stretch. In the original Harry Potter series, we see the Gryffindor class transfiguring all kinds of animals - hedgehogs into pincushions, desks into pigs, mice into snuffboxes. We also know that it's possible for a human to be transfigured into an animal (ferret-Malfoy) and for animals to be conjured (like the birds Hermione set on Ron). So it would seem fairly straightforward for Credence to have picked up a random chick out of the kindness of his heart, and for Grindelwald to transfigure it to sell him on a lie.
It's certainly not out of the question for Grindelwald. After all, the wizard didn't shrink from murdering a toddler just so he could use a house, why would he be concerned about a little transfiguration to back up a useful lie? The truth will surely be revealed in Fantastic Beasts 3.