In ancient times, a seal was a mark of ownership that authenticated the owners
possession of an item.
For example, it was common to seal a scroll or letter with wax and stamp it with
the signet ring of the one sending the scroll so as to verify the authenticity of the letter.
Circumcision was Gods mark of ownership on the Jew, to show that he belonged to God.
Note: New Testament believers are said to be sealed with the Holy
Spirit.
Ephesians 1:13 - And you also were
included in Christ when you heard the word of truth, the gospel of your
salvation. Having believed, you were marked
in Him with a seal, the promised Holy Spirit