Credence didn't understand. He loved Grindelwald so much. Credence had helped the wizard, even after he had turned out to be wearing another person's face and after he had betrayed Credence. So why was Grindelwald kissing someone else?

Basically Grindelwald wins Credence's trust back post film, with apologies, declarations of love, and promises of power. Grindelwald manages to separate the obscurus from Credence and find a way to use it to take over the world. Along the way, Grindelwald either reconciles with Dumbledore, who takes him prisoner, and they end up together. Credence isn't thrown away, but he definitely gets pushed aside a bit.