What does Mark 15:26 mean?

It is standard procedure for crucifixion victims to be paraded about with a sign announcing their convicted crime. Once they are secured to the cross, the placard is attached above them to dissuade bystanders from committing the same offense. The Jewish people are told in no uncertain terms what will happen if they attempt to rebel and anoint a king other than Caesar.

The Jewish leaders, the Sanhedrin, convict Jesus of claiming to be the Messiah and the Son of God, a crime worthy of death (Matthew 26:66). "Messiah" is from the Hebrew Mashiyach, presented as Christos in Greek, both meaning "anointed." This means nothing to the Romans, but the rough translation is "King of the Jews." If Jesus is "King" of the Jews, as the Romans would interpret it, the Sanhedrin members are committing treason against their own King and blasphemy against their God. If Jesus isn't King of the Jews, He's no threat to Pilate or Caesar.

The sign is Pilate's political backhanded slap at the Jewish leaders (John 19:19–22). He has the sign written in Aramaic, Latin, and Greek, and keeps the wording despite the protestations of the chief priests. It's unclear if Pilate thinks Jesus really could have pushed back the Romans and established Himself as king, or if Pilate is disgusted by the teachers and enforcers of the Jewish law who demand the crucifixion of an innocent man. Undoubtedly, he recognizes that this gentle teacher who heals and tells His followers to obey the law and love others is a far better choice for Jewish king than the corrupt officials. The Jewish leaders and soldiers call Jesus "King of the Jews" to mock Him (Mark 15:18, 32). Pilate does it to mock the leaders.

Nevertheless, the sign over Jesus' head (Matthew 27:37) is important. Despite the fact the Sanhedrin denies Jesus is King of the Jews or the Messiah (John 19:15), the legal charge designates Jesus is executed because He is King of the Jews. He dies because He is the Messiah.