John had earlier seen an angel mandated to seal the servants of God on their foreheads (Revelation 7:3). In a way not detailed to us (see commentary on Revelation 7:3), the Lord placed a public mark on the foreheads of those Christ ransomed from sin and Satan, so that one and all could see that these persons were God’s possession.
4 They were told not to harm the grass of the earth or any green plant or any tree, but only those people who do not have the seal of God on their foreheads.