Let's take a look at the armour of God:
Ephesians 6:14-17
Stand therefore, having girded your waist with truth, having put on the breastplate of righteousness, and having shod your feet with the preparation of the gospel of peace; above all, taking the shield of faith with which you will be able to quench all the fiery darts of the wicked one. And take the helmet of salvation, and the sword of the Spirit, which is the word of God. praying always with all prayer and supplication in the Spirit, being watchful to this end with all perseverance and supplication for all the saints.
Girded your waist with truth - this means to secure with a belt or band. The belt was an essential part of the equipment, passing round the loins and at the end of the breastplate (in later times supporting the sword), it was especially used in keeping other parts in place, and aided with the freedom of movement.
The verse tells us that we must be girded with truth; meaning we must be girded with His word, for His word is truth.
It's significant that it's mentioned first in the list because Jesus said He is the way, THE TRUTH, and the life. If we don't accept Jesus as the Christ and as our Saviour, then we can forget about anything else He said; but if we acknowledge Him as such, and as our Lord - then everything He says has to be held in highest regard! (He said the man that builds his house on the rock is the one that hears and does His word).
Everything is therefore kept together by truth (by His word); It is the starting point of our armour, for it holds all the other areas of our lives in place.