The rest of verse13 makes it clear that we will be in a battle; that we will see what happens after the battle: if we have God's armor on we can be assured that we'll still be standing firm; that it is but one of many battles; and, that it is not the whole war. We as Christians must understand that battles will come, many of which we've probably already faced without even realizing we were in them! Those were the times when we either succumbed to temptation and felt ashamed afterwards because of our "failure," or when we enjoyed a real blessing because we were obedient without realizing God had clothed us with armor we didn't realize we were wearing! Only in heaven will we get the full picture of our lives on earth and we'll be amazed how often God brought us through many struggles. And we'll also realize how unprepared we were so many times because we neglected to dress ourselves with God's armor.
The first piece of armor Paul speaks of is the "belt of truth." Truth is who Jesus is, John 14:6, and what best describes God's Word, John 17:17. If we are going to face an enemy who is known as the "father of lies," John 8:44, we need to clothe our hearts and minds with God's truth. There are many passages in the Scriptures that speak of the deceitfulness of Satan. And a Christian who isn't clothed with God's truth will fall victim to the enemy's lies. Truth allows us to grow strong because we are believing what God said. We are able to help other believers grow because we will teach them the truth that we ourselves have learned. Truth not only protects from lies it also gives us great joy when we see all that God has prepared for us!
Learn the truth by studying the Word. This is how you clothe yourself with truth. You can't just expect someone else to know the truth and you just rely on them standing for truth. We are all called on by God to wear this piece of the armor every day. You will be faced with deceit all around you. God's truth in your heart will give you the ability to stand against the enemy.