Do not labor for the food which perishes, but for the food which endures to everlasting life, which the Son of Man will give you, because God the Father has set His seal on Him. John 6.27
Nowadays, people use their signature, but in ancient times kings set laws with a royal seal ring.
The baptism with the Holy Spirit is the Seal, the mark of God’s ring on His servants. All of hell sees and recognizes this mark on the faithful. Because of this, evil has no power over them.
However, those who do not have this Seal, even though they believe in God, are still subject to the successful onslaughts of hell.