Mark 16:5

And entering into the tomb, they saw a young man sitting on the right side, arrayed in a white robe; and they were amazed.

The young man ... Mark surely intended that we should understand that this was an angel of heaven. Some people refuse to believe in the existence of angels, having been poisoned by the leaven of the Sadducees; and, like the Sadducees, they "do greatly err, not knowing the Scriptures, nor the power of God" (Matthew 22:29). Again, from the words of Cranfield:

Here a protest must be made against the widespread tendency to dismiss angels as mere pious fancy ... The purpose of angels at the tomb was to link the actual event of the resurrection and the women. Human eyes were not permitted to see the event of the resurrection itself... The angels as the constant witnesses of God's actions saw it ... By their testimony the resurrection ... was made known to men.[3]