From John Piper on his website:
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. (emphasis mine)
Read the whole article here.