Marvel not - Do not wonder or be astonished at this.
The hour is coming - The “time” is approaching or will be.
All that are in the graves - All the dead, of every age and nation. They are described as “in the graves.” Though many have turned to their native dust and perished from human view, yet God sees them, and can regather their remains and raise them up to life. The phrase “all that are in the graves” does not prove that the same particles of matter will be raised up, but it is equivalent to saying “all the dead.” See the notes at 1 Corinthians 15:35-38.
Shall hear his voice - He will restore them to life, and command them to appear before him. This is a most sublime description, and this will be a wonderful display of almighty power. None but God can “see” all the dead, none but he could remould their frames, and none else could command them to return to life.