A voice came from heaven: Compare Matthew 3:17; 17:5; Mark 1:11; 9:7; Luke 3:22; 9:35.
God had already brought glory to himself when Christ entered the world, and through the work of Jesus. This showed God's power to the world (John 1:14).
I will glorify it again: The final display of glory would come at the cross (see 12:23; 17:1). In John's Gospel, the cross is an event of glory as Christ was "lifted up" and glorified there (12:32).
Jesus was prepared for crucifixion like a king coming to his coronation. The cross was like a throne (see 19:19–22). They buried Jesus like a king.
The time of his glorification included all the events from his arrest through his resurrection.