John had already mentioned this altar in Revelation 6:9. The souls of those martyred for the sake of the gospel cried out to God from under this altar for God to pour out his vengeance upon their murderers. With his approach to this altar, this angel begins God’s response to the martyrs’ cry.
3 And another angel came and stood at the altar with a golden censer, and he was given much incense to offer with the prayers of all the saints on the golden altar before the throne,