As Jesus speaks about his approaching death, he declares that judgment has come upon this world and that 'the prince of this world' will be cast out.
The expression describes a spiritual adversary whose apparent power is overturned through the very event that looks like Jesus' defeat.
Jewish apocalyptic and early Christian literature can portray evil spiritual powers exercising limited influence within the present age while remaining subject to God's ultimate judgment. John's Gospel elsewhere identifies the devil as the father of lies and later refers again to the 'ruler of this world.' The title does not imply an equal rival deity or legitimate ownership independent of God.
The verb 'cast out' presents the cross as judgment upon this hostile ruler.
The ancient spiritual worldview helps explain why Jesus can describe his death simultaneously as suffering, glorification, judgment, and victory.
John reverses appearances: the moment when worldly and spiritual powers seem to overcome Jesus is the moment in which their ruler is judged and expelled.