Truly I tell you: This introductory phrase shows that this promise is important, but people debate what the promise means. It might mean that some disciples will:
witness the resurrection of Jesus from the dead (chapter 16);
experience the coming of the Holy Spirit upon the church (Acts 2);
witness the destruction of Jerusalem in AD 70; or
witness the transfiguration of Jesus as a preview of God's coming kingdom.
Mark places this promise right before the transfiguration story (Mark 9:2–8). This suggests he likely considered the transfiguration as its fulfillment (see 2 Peter 1:16–18).