What if when Graves arrives at the church he realizes right away that Credence is the Obscurus. Credence is at his most vulnerable and begging for help and the way Mr. Graves is looking at him...so full of wonder and awe at the power Credence possesses. To be told that he's a miracle. There is no question that he would follow Mr. Graves anywhere.

There is no betrayal, Graves wins and runs away with his prized and loyal Obscurus (and he never ever turns into Johnny Depp)