In days of old, a king would seal a special letter with his royal seal stamped
This let everyone know that the letter was truly from the king, contained
his word, and was to
be guarded and kept safe.
In the same way, (though spiritually, and
not with wax!) each new believer is "sealed"
with the Holy Spirit, to show the truth
and genuineness of his new life, and
as a promise from God of what is
to come in the future, when
Jesus comes back