The title reveal of Fantastic Beasts 3 has sent fans into a whirlwind of trying to figure out what exactly Dumbledore's secrets are - however, it is much more possible that the newest addition to the Harry Potter universe is about Grindelwald's lies. J.K. Rowling has already included a very detailed history of the Dumbledore family courtesy of Harry Potter and the Deathly Hallows. While the final installment of the Harry Potter franchise does include some information about Dumbledore's past, much more about the Hogwarts headmaster's history is included in the books.
So far Rowling has created a formidable antagonist in Gellert Grindelwald. While Voldemort and Grindelwald are comparable in terms of power and being a danger to the wizarding world, Grindelwald relies more on persuasion and manipulation rather than unforgivable curses. Fantastic Beasts 2 showcases Grindelwald's cunning during his artful call to action at the Paris rally in the Lestrange mausoleum. Grindelwald's words are compelling enough to draw Queenie into his movement, possibly one of the saddest losses of Fantastic Beasts 2. Since the previous movie gave audiences a taste for what Grindelwald is capable of in regards to his influence, it's highly possible that "Dumbledore's secrets" may not be what viewers have in mind.
Grindelwald will do anything he can to smear Dumbledore's name in order to keep followers on his side when Dumbledore's opposition inevitably comes his way. Since there is already plenty of information provided about Dumbledore, his past, and even Dumbledore's relationship with Grindelwald, it seems unlikely there are that many of Dumbledore's secrets left to be revealed. Therefore, it is entirely possible that Grindelwald is lying to Credence about being a Dumbledore. From the beginning, it is clear that Grindelwald flexes his power best when he is using other people, including Credence. As well, it was divulged in Fantastic Beasts 2 that Dumbledore can't fight against Grindelwald due to a blood pact the two made when they were young. Grindelwald is using the blood pact as a temporary measure in order to keep Dumbledore away for as long as it is feasible. This is why Dumbledore sent Newt Scamander to search for Credence and Grindelwald in the first place. Since neither can directly engage in combat with the other, Grindelwald will have to fight much more covertly to get Dumbledore out of the way.
Making Credence Dumbledore's brother could repeat the previous Fantastic Beasts mistake by breaking the Harry Potter canon. Dumbledore's father Percival was sentenced to Azkaban for life in 1890 after violently retaliating against a group of muggle boys who assaulted Ariana, the youngest Dumbledore, and Albus' sister. Ariana suppresses her magic after that and was prone to fits, resulting in their mother Kendra's death in 1899. Credence is listed as being born "around" 1901, which doesn't fit into this timeline whatsoever.
It's not entirely certain what ulterior motives Grindelwald has in lying to Credence about his heritage as a Dumbledore. It could be to keep Credence on his side so that he can harness Credence's obscurus for destruction and mayhem against Albus. Hopefully, this will all be revealed in Fantastic Beasts 3, though it's highly unlikely considering Rowling's habit of holding out information until the last minute. Retconning the Credence/Dumbledore plot twist could only help the series. With luck, Dumbledore's secrets won't be a re-hash of the information given in the books, but rather a focus on the lies that Grindelwald tells about Dumbledore to continue his sinister plan.
