Judas' moral state becomes clear with the later disclosure by John that Judas was stealing and, by that theft, was depriving others of much needed charitable contributions.This deliberate moral state - the repeated act of theft - needs to be considered in juxtaposition to the stated question, as it paved the way for Satan's entrance.
Judas had not been resisting Satan over the matter of secret theft.
This paved the way for much greater sin, later. Then, Satan came in like a flood and took him over completely.