What does Mark 15:18 mean?

Romans of this era neither understand nor respect Jews. The Greco-Roman religion includes a pantheon of gods from which to choose. Cities, households, and individuals can decide for themselves whom to worship. And the sitting emperor is considered a god-in-training who will reach full deity upon his death.

Jews, in contrast, had long ago given up their Baal worship and Asherah poles. Since their return from captivity in Babylon, they've worked hard to worship God and God alone. The Romans think the Jews' devotion to a single, invisible God may as well be atheism. Not only is it disrespectful to the emperor, it's backwards and unsophisticated in their eyes.

At this time, the Jews are scattered all over the Roman Empire, but live mainly in Judea, Galilee, and Perea. Pilate is prelate over Judea, the area around Jerusalem. Herod Antipas is tetrarch over Galilee, north of Samaria, and Perea, east of Judea. Despite Herod Antipas' assumed title, the Jews have no king. They haven't had one since Herod the Great who died a few years after Jesus was born (Matthew 2:19). There won't be another king over Jewish territories until AD 41 when Caesar consolidates Herod the Great's territories under Herod's grandson Agrippa I.

Much of what happens to Jesus is as much about mocking the Jews as it is punishing Jesus. The guards crown Jesus with thorns and hail Him as "King." Pilate presents Jesus to the Jews as king (John 19:14) and writes "King of the Jews" on a sign above His head on the cross (Mark 15:26). The sacrificial, loving leadership that Jesus is displaying is so foreign to the Romans that they don't see the power in it (Mark 10:42–45). Neither do the Jews. They insist to Pilate, "We have no king but Caesar" (John 19:15).