A.T. Robertson Commentary John 13:27

"And after the sop, then entered Satan into him. Jesus therefore saith unto him, What thou doest, do quickly." — John 13:27 (ASV)

Then entered Satan into him (τοτε εισηλθεν εις εκεινον ο Σατανας). The only time the word Satan occurs in the Gospel. As he had done before (13:2; Luke 22:3) until Christ considered him a devil (6:70). This is the natural outcome of one who plays with the devil.

That thou doest, do quickly (Hο ποιεις ποιησον ταχειον). Aorist active imperative of ποιεω. "Do more quickly what thou art doing." Ταχειον is comparative of ταχεως (John 11:31) and in N.T. only here, 20:4; Hebrews 13:19,23. See the eagerness of Jesus for the passion in Lu 12:50.