“The man who has accepted it” is a once and for all action, a decisive act.

“seal” (KJV) or “certified” (NIV) was much used in earlier times because there were many who could not read.  A seal indicated possession but came to mean authentication, a personal guarantee by the owner of the seal that the thing sealed was the genuine article.  Luther wrote, “Among men nothing is safer and more certain than that which is given under one’s hand and seal.” If you received a sealed letter, you could be certain that it was the real thing and untouched by any other.

When a man accepts Christ he is not entering into a human relationship; he is acknowledging the truth of God as revealed in Jesus.  He is proclaiming to all that what God has said through His Son is true.