Stand therefore, having fastened on the belt of truth

This verse is telling Christians to be stable people. Nothing is worse than an unstable Christian. This kind of Christian allows Satan to get a foothold. Once he has a foothold he uses that person to cause harm to the church, to others and ultimately to himself. If you are to fight this spiritual battle, you must be stable. In order to be stable, two things must happen (1) you must have the armor of God and (2) you must use the armor of God. Having the armor of God comes about through faith. Through faith, God unites us to Christ. In this union, we are clothed with God’s armor. Yet we must also make use of this armor. We will see this twofold dynamic in every piece of armor.
The words “having fastened” are interesting in the Greek. It is an aorist participle meaning that you already have fastened the belt of truth, but it also has an imperatival sense. This means that you must fasten the belt of truth. In other words, God has given you the belt of truth, but you must use it.   
In a Roman soldier’s armor, the fastening of the belt prepared him for battle. The belt held things together. When he fastened it he drew in his tunic so that it would not impede him in battle and it also firmly held his sword in place. Paul is saying the same thing for us. Truth prepares us for battle and gives us confidence. It holds the spiritual armor in place and safeguards us against daily problems.
The “truth’ that is being referred to is twofold. First it refers to the truth revealed in Scripture and second it refers to truthful character. The idea here is that we are given truth in our union with Christ (John 14:6), but we must use this piece of armor by living truthful lives.          
The Christian is to know the truths of Scripture well. We have seen that one of the schemes of Satan is false doctrine. Satan perverts the truths of Scripture for at least two reasons (1) to keep people out of the kingdom and (2) to cause people in the kingdom confusion and lack of security. Those in the kingdom are to be mature and thinking people. We are to base our thinking and emotions on the truths of Scripture. If we don’t, we will be unstable. Paul says this in Ephesians 4:14,
14 so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes. (Eph 4:14 ESV)

True doctrine is essential for spiritual battle.
In addition to knowing doctrine, we are to live truthful lives. The truths of Scripture are to be worked deep into us so that they produce integrity. There is a cause and effect relationship between true doctrine and truthfulness in life. The Holy Spirit uses the truths of Scripture to transform our characters. In order to resist the devil, we need doctrinal truth and truthful lives.