Every verse deserves a volume of commentary and this verse is certainly worthy of several editions. There's a lot here but, as usual, I only comment briefly with hopes that what I share God will use to enrich your own understanding of His Word.
Paul makes clear with the word "therefore" that there is a proper response to the knowledge of just who your enemy is. You need the whole armor of God to engage this enemy in battle. This is not a "chose your own" wardrobe and select what implements you'll use and what you'll leave behind. God is the Commander-in-Chief and what He says, goes! He wants you to be dressed fully and, therefore, equipped to face not only the enemy but also to use your armor in winning others to Christ and providing a certain amount of protection for them.
Please note that Paul speaks of our being "able to resist the enemy..." Does that not remind you of James 4:7 NLT, "So humble yourselves before God. Resist the devil, and he will flee from you"? Resisting Satan is our responsibility but it is a fruitless effort if we rely on our own strength to do so! Our resistance is to come from a humble spirit that is clothed in the full armor of God. We cannot defeat Satan. God already did! Note Hebrews 2:14 and 1 John 2:8. But we can cause him to flee from us and from those we are witnessing to. Consider the rebuke of evil spirits throughout the Gospels and think of Paul's rebuke of the sorcerer in Acts 13:6-10 in order that Paul could win the governor to the Lord.
The "time of evil" has been around for a long time. We're living in very evil days and more than ever we need to be clothed in the full armor of God if we are going to stand strong for God and win many to our Savior.