Why discuss "salvation" when it is assumed Paul knows he is talking to believers who are already saved?! The answer may be seen in understanding two important truths:
First, salvation should always be on our minds, not that we need to be saved again and again, but more so that salvation, like the cross, is something we should never forget! Hence, the protection of the head where we keep our thoughts! Keeping our salvation on our minds reminds us that we were saved from a life of sin and we don't want to go back to that again. Salvation also teaches us that we've been given a new life that we should want to live! Also, salvation reminds us that we just weren't saved to forgive our sins. Salvation also teaches the next truth:
Second, salvation has three tenses. PAST TENSE - We were saved from the penalty for our sins. Romans 6:23 reminds us that "The wages of sin is death..." and John 3:16 reminds us that those who "...believe on Him will not perish..." Our sins were dealt with and we were forgiven. PRESENT TENSE - We are constantly being saved from the power of sin! Romans 6:10-11 NLT says, "When He died, He died once to break the power of sin. But now that He lives, He lives for the glory of God. So you also should consider yourselves to be dead to the power of sin and alive to God through Christ Jesus." We no longer need to say "Yes" to sinful temptations. Christ has given us the power to say "NO" to sin! FUTURE TENSE: we will one day be saved from the presence of sin. Psalms 103:12 NLT tells us, "He has removed our sins as far from us as the east is from the west." Also, Philippians 3:21 NLT - "He will take our weak mortal bodies and change them into glorious bodies like His own..." As He is without sin, so we will be in eternity!!
Now, knowing that we have such a complete salvation is a strong part of the armor to protect us from Satan's power.