Dr David Petts explains the meaning of the Holy Spirit as a Seal in the second of his talks on the Holy Spirit and the Future.
Scripture references:
2 Corinthians 1:21-22
Ephesians 1:13-14
Romans 4:11
Jeremiah 32:10ff
Genesis 41:41-44
Isaiah 29:11
Romans 8:18-23
Ephesians 4:30
1 Corinthians 15:50-54