And take the helmet of salvation and the sword of the Spirit, which is the word of God. -Ephesians 6:17 NASB
The moment you get saved by accepting Jesus Christ as your Lord and Savior, He gives you the helmet. The helmet protects you from a deadly force hitting your head, causing you to be unconscious. It would only take one blow to the head to kill you. That is why Jesus gives you the helmet right after you get saved. It also protects you from believing false doctrine that you can lose your salvation.
The sword is used as an offensive and defensive weapon. It destroys every false doctrine—arguments against what you hold to be sound doctrine to convince you to adopt another version of the truth. Jesus used the sword—the Word of God—in Matthew 4:1-11 against Satan. Not only is the sword for striking your enemy, but it also blocks false teachings from hitting you by saying like Jesus did “It is written”, meaning, you stop the misinterpretation that you are getting by showing them in another passage that they are twisting the meaning of the passage.
The helmet and the sword will protect you from false doctrine. It is vital to always have them on you. The sword is also for destroying a false narrative given to you by a false teacher about the person of Jesus Christ, having the Scripture clarify in another passage the true meaning of the verse in question, so that they may repent and get saved.