Revelation 9:4
ESV - 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.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
It is the Grace of God.
Whilst the question concerning what the Mark of God actually is, belies any distinction as such. The Bible, as far as I am aware, does not give any clear distinction concerning what the Mark of God is.
That said of course, there are many thoughts about what this mark is and what it might mean. Numerous speculations have been made over time and is heavily subject to guess work. However, I am certain that the word of God contains the answer to this question that has plagued scholars and ministers alike. It may be, that God has reserved this revelation of Truth concerning the identity of HIS MARK, until the time that he has designated for its revelation. This may be made known only when the world has moved closer to the time of our Lords coming.
All told, this is an interesting subject that requires more study in order to gain clarification as to what the Mark of God actually is!