In the Book of Revelation, there are a number of verses that talk about the seal of God. One example is found in the following verse.
Revelation 9:4 "They were commanded not to harm the grass of the earth, or any green thing, or any tree, but only those men who do not have the seal of God on their foreheads."
But what is this seal of God?
Look at the following verses and I think that the only conclusion that can be made is that this seal of God is indeed, the Holy Spirit.
Ephesians 1:13 "In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise."
Ephesians 4:30 "And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption."
2 Corinthians 1:22 "Who also has sealed us and given us the Spirit in our hearts as a guarantee."
Timothy 2:19 "Nevertheless the solid foundation of God stands, having this seal: "The Lord knows those who are His," and, "Let everyone who names the name of Christ depart from iniquity."