What does Mark 8:38 mean?

"Adulterous" is from the Greek root word moichalis. Metaphorically, it means to reject God and chase after what the world offers. The term was used extensively of the Jews in the Old Testament when they worshiped foreign idols.

"Glory" is from the Greek root word doxa and means splendor, brightness, and exaltation. Scholars disagree on the time of Jesus' glory; some say it will be at the rapture (1 Thessalonians 4:13–18) while others say it refers to Jesus' second coming at end of the tribulation (Revelation 19:11–21). Since it is at the second coming that Jesus officially returns to earth in glory, the verse most likely refers to the latter event.

In the course of three chapters, it is affirmed that Jesus is the Christ twice (Mark 8:29; 9:2–8); Jesus reveals His coming suffering, death, and resurrection three times (Mark 8:31; 9:31; 10:33–34), and tells the disciples they must be humble many times (Mark 9:35, 40–41; 10:15, 24–25, 42–45). The two times Jesus show the disciples a hint of His glory, He tells them to tell no one (Mark 8:30; 9:9), but He never gives a similar order about His coming disgrace and death. In fact, He calls His followers—at least metaphorically but often literally—to join Him on the cross (Mark 8:34).

Jesus warns the disciples that their idea of what is shameful and what is honorable is completely warped. They cannot share the glory of the Son of Man from Daniel 7 if they reject the Suffering Servant of Isaiah 52:13—53:12. They will understand eventually. Although after the crucifixion they will hide in fear (John 20:19), soon they will greet prison and beatings with joy, as a sign they are worthy to suffer for their Savior (Acts 5:40–41).