The Belt of Truth is the first element of the Full Armor of God.
In the ancient world, a soldier's belt not only kept his armor in place, but it might be wide enough, as a girdle, to protect his kidneys and other vital organs. Just so, the truth protects us. Practically applied to us today, you might say the Belt of Truth holds up our spiritual pants so that we're not exposed and vulnerable!
Jesus Christ called Satan the "father of lies", and deception is one of the Enemy's oldest tactics. We can see through Satan's lies by holding them against the truth of the Bible. The Bible helps us defeat the lies of materialism, money, power, and pleasure as the most important things in life. Thus, the truth of God's Word shines its light of integrity into our lives and holds together all of our spiritual defenses.
Jesus told us "I am the way and the truth and the life. No one comes to the Father except through me." (John 14:6, NIV)
Next we'll strap on the Breastplate of Righteousness ...