The first piece of armor on the list is the "belt of truth." Consider the image of the high priest. We might note the sash around his waist and think "that doesn't seem like much of a belt!" Examining the Ephesians passage we do not find a "belt of truth" actually mentioned. Instead, we find this: "Stand firm therefore, HAVING GIRDED YOUR LOINS WITH TRUTH...". The "belt of truth" is simply the common phrasing and imagery used when discussing the armor of G-d. The text of Ephesians 6 does not include those words nor does it even contain the Greek word for "belt" (zone- pronounced dzonay). Instead, Paul's words translate literally as "girding the loins of you in truth."
When Adam and Chavah sinned, their sin uncovered them. The first thing they did afterward was to try to cover their nakedness. They saw the truth of their circumstances and attempted to cover themselves with fig leaves (Genesis 3:7). That covering was inadequate, and G-d later gave them garments of skin (Genesis 3:21). The skins were a physical covering that pictured their spiritual fallen state. Adam and Chavah were uncovered as a result of sin so the opposite should be true: righteousness covered them before their sin.
In the case of the Levitical priesthood, Exodus 28:42 tells us that they are to have linen breeches upon their loins to cover their nakedness. Exodus 28 pictures a spiritual covering using the priests' linen breeches: a covering of G-d's righteousness!
Perhaps the ultimate fulfillment of that linen covering appears in Revelation 19-
It was given to her to clothe herself in fine linen, bright and clean; for the fine linen is the righteous acts of the saints. (Revelation 19:8)
Revelation 19:8 tells us the bride of the Lamb is given fine linen with which to clothe herself. Note that she is not passively being clothed, but she is "to clothe herself." The linen is "the righteous acts of the saints."
The linen/"righteous acts" which G-d gives us are the good deeds [mitzvot] of Scripture prepared beforehand for us to walk in them (Ephesians 2:10). Scripture is Truth (John 17:17), so the bride is clothed in the Truth of G-d's Word. Messiah is the Word of G-d made flesh (John 1:14), so we are clothing ourselves in Him!
This fits with Paul's encouragement to the believers in Galatia:
"For all of you who were baptized into Christ have clothed yourselves with Christ." (Galatians 3:27)
Messiah also said that He is the truth (John 14:6). If we clothe ourselves with the truth, then we can again say that we are clothing ourselves with Him.
Note that Paul's words in verse 14 are in all capital letters: "HAVING GIRDED YOUR LOINS WITH TRUTH." The NASB uses the convention of capitalizing passages that are quotes from the Tanakh. This passage is a quote from Isaiah 11. That chapter provides an excellent description of Messiah that includes a description of a "belt."
Also righteousness will be the belt about His loins, And faithfulness the belt about His waist. (Isaiah 11:5)
The Hebrew word translated here as "belt" is ezor. Scholars alternately translate it as "girdle," "waistband," or "belt" throughout the Tanakh. Messiah girds himself with righteousness and faithfulness. When we clothe ourselves with Him, then His righteousness and faithfulness are upon us.
Observant readers will notice that "truth" is not mentioned in this Isaiah passage. The Septuagint translation of this passage, however, does include the word truth:
And righteousness will be tied around his loin; and with truth being wrapped around his sides. (Isaiah 11:5 -LXX)
Speaking of righteousness, the breastplate of righteousness is next on our list.