In Bible times a seal was the way a person identified something that he owned. For example, business men would purchase timber in Lebanon. When they purchased it they stamped it with their seal. The logs were then floated down the coastline of the Mediterranean Sea to the port where the buyers could collect their logs. Any log with their mark of ownership on it belonged to them.
The Christian is sealed with the Holy Spirit—he has God’s mark of ownership, and God does not give up His own.