The helmets referred to in the Bible are for protection in mortal battle. A blow to the head with a mace or a bludgeon would crush your skull and kill you.
So when Paul says that we should put on “for a helmet the hope of salvation” he means that there are blows that come to our spiritual life that could destroy us, if we were not protected by the hope of salvation.
The hope of salvation—that we will not perish but obtain eternal life in the presence of Christ—absorbs the blow and keeps it from killing us. Blows still come in war and in life. Helmets don’t prevent blows. They just prevent them from destroying us.
One of my clearest experiences of how this works was in December, 2005. The urologist said there was an irregularity in the prostate. He would like to do a biopsy. When? Right now. I’ll be back in a few minutes with the instrument. You can wait on that table.
In those ten minutes of his absence I felt a blow. He thinks I have cancer. He wants to do the biopsy instantly. As the blow descended on my head, the Lord positioned my helmet with loving firmness so that it wouldn’t fly off.
This is the “hope of salvation” that he gave me: “For God has not destined us for wrath, but to obtain salvation through our Lord Jesus Christ, who died for us so that whether we are awake or asleep we might live with him” (1 Thessalonians 5:9-10).
He brought this text to my mind as I sat on the table awaiting the biopsy. It did its work. It fixed hope on my head. It put brass between me and the blow.
I didn’t notice till later that the “for” at the beginning of the promise in 1 Thessalonians 5:9 (“For God has not destined us for wrath...”), was connected to the helmet of 1 Thessalonians 5:8: “...having put on for a helmet the hope of salvation. For God has not destined us for wrath, but to obtain salvation.”
In retrospect, God covered my head with the promise that this blow was not his wrath. He positioned the helmet of hope perfectly without my even thinking of helmets. I simply thought: This is not wrath; and if I live, I live with Christ, and if I die, I also live with Christ. With that he covered my head.
So go to the arsenal of God’s word and get your armor. The blows are going to come. Without a helmet they will crush your skull. God has a helmet of hope fitted for your head. Put it on.