One theory states that Gellert Grindelwald (Johnny Depp/Mads Mikkelsen) didn't lie about Credence's (Ezra Miller) identity in Fantastic Beasts 2 - he genuinely believes he is Aurelius Dumbledore. Fantastic Beasts: The Secrets of Dumbledore promises to explore Albus Dumbledore's (Jude Law) past as well as Credence's identity and his relationship to the great wizard after the previous movie left many wondering just how much of what Grindelwald said is true. While Grindelwald is known for manipulating people into fighting his battle (having done just that to Credence in the first Fantastic Beasts movie), when it comes to Credence's identity, Grindelwald is following the extensive research he's done over the course of Fantastic Beasts 2.
Grindelwald was a natural-born Seer and had a vision of an Obscurial killing the man he feared the most - Albus Dumbledore. This is why he traveled to New York (assuming the identity of Colin Farrell's Percival Graves), looking for the reported Obscurial wreaking havoc in the city. Grindelwald didn’t think Credence could be the Obscurial (in fact, he was surprised and impressed when it was revealed), as he was initially looking for a child under 10 years old, as no witch or wizard possessed by an Obscurus makes it past the age of 10. However, there was one exception: Ariana Dumbledore.
When Grindelwald discovered Credence was in fact the Obscurial, he connected the case to Ariana's and he started suspecting that Credence was also a Dumbledore (as Dumbledores are very powerful wizards and can arguably survive an Obscurus for longer). The involvement of Dumbledore and his protégé Newt Scamander (Eddie Redmayne) in the events of New York only cemented Grindelwald's suspicions. No one apart from Albus and Aberforth (Richard Coyle) knew about Ariana's Obscurus, so Grindelwald - who was once very close to Albus and became at least partially responsible for Ariana's death - was the only person who could deduce Credence's identity.
Grindelwald didn't just follow a deduction when he revealed Credence's identity as Aurelius Dumbledore. In Fantastic Beasts 2, he was researching possible candidates for his identity when he stumbled upon Irma Dugard (Danielle Hugues) and the story of Corvus Lestrange. Irma most likely knew that Corvus had drowned and that Credence's real name was Aurelius (the youngest Dumbledore child, unacknowledged by the family and sent to live in America). Irma herself knew this story - or at least part of it - from Aurelius' aunt, who was traveling with her to the United States. A scene from the Fantastic Beasts 2 script that didn't make it into the movie shows the two women familiar with each other after the long trip. As the ship sinks, Credence's aunt calls out: "Irma? They want us to put on life jackets!".
It's still unclear whether Grindelwald is right about Credence's identity as Aurelius Dumbledore. The Fantastic Beasts 2 Credence twist had a major effect not only on the prequel franchise but on the Harry Potter canon as a whole, and Fantastic Beasts 3 needs to clarify Credence's identity in order to avoid the negative feedback the second installment received. But as of Fantastic Beasts 2's ending, Credence believes he is Aurelius, and so does Grindelwald.
