A great post from Grace to you.
To protect us from Satan's crushing blows, Paul tells us to "take the helmet of salvation." Now considering all he's been telling us so far, he was not saying, "Oh, by the way, go get saved." Paul was addressing believers. Unbelievers don't have to put on spiritual armor. They aren't even in the battle. Satan doesn't attack his own forces.
In 1 Thessalonians 5:8 Paul describes the helmet of salvation as "the hope of salvation." That implies Satan's most fierce and powerful blows are directed at the believer's assurance and security. Therefore Paul was encouraging believers to have confidence in the salvation they already possess. He knew that doubting their security in Christ would render them ineffective in spiritual warfare--just as a blow to the head renders one's physical body incapable of defending itself.
As a believer, you should have the assurance that you are secure in Christ. If you don't, you haven't put your helmet on, and that makes you vulnerable to discouragement and doubt. Romans 8:29-30 assures us that all whom God justifies He sanctifies and glorifies. No one is lost in the process.
Jesus said, "My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they shall never perish; and no one shall snatch them out of My hand" (John 10:27-28). That's a wonderful promise. So don't let your enemy rob you of the joy and assurance of knowing you belong to Christ, for the Lord will never let you go (Heb. 13:5).