It’s noted earlier in the film that there is a strong connection between the Dumbledore clan and the fiery birds (as demonstrated by Albus’ relationship with Fawkes in the Harry Potter movies), and the attraction of a phoenix to Credence solves a major mystery.
The reveal I’m referring to, of course, is the fact that Credence Barebone — who has never known anything about his parents — is actually a member of the Dumbledore family.
This information is shared with the confused young man shortly after he decides that he is going to follow Grindelwald (Johnny Depp) and his dream of wizard supremacy, and demonstrated with the help of a phoenix.
Fantastic Beasts: The Crimes of Grindelwald Ending | What Happens, and What It Means