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.

>Then saith one of his disciples, Judas Iscariot, Simon's son, which should betray him, Why was not this ointment sold for three hundred pence, and given to the poor? This he said, not that he cared for the poor; but because he was a thief, and had the bag, and bare what was put therein. John 12 5,6 KJV.

The covetousness is apparent, the very calculation of value (and lost revenue to the thief) gives him away. This precipitated something within him. Was his betrayal a matter of recuperating the lost opportunity ?

What a shocking state he was already in !