We are saved by grace and grace alone.
The belt of truth
Picture a Roman soldier in the midst of battle, not wearing his belt (because in the hurry before battle he forgot to put it on), jumping to and fro to avoid being pierced by the enemy, holding up his shield with one arm and his sword with the other, all the while trying to hit his opponent.
What do you think the breastplate, protecting his heart and other vital organs, will do? Most probably it will swing from left to right, away from his body and back, but seldom in place to do what it was for: to shield.
At times he will be inclined to press his breastplate against his body, but if he does so with only one arm he will either be unable to lift his shield or use his sword. And if he tries to keep it in place with both arms, he will be giving up both the extra protection of the shield and his sword to ward off enemies!
So it is clear how very important it is that he wear his belt. The belt seems to us like a very passive instrument, his sword or his shield appearing much more active, and therefore much more important in battle. But without his belt he is practically debilitated, without it he is a danger to himself and his fellow soldiers, and certainly no danger to his adversaries.
He is useless without his belt.
Paul says to put on the belt of truth. Peter says (1 Peter 1:13) that God's elect should "brace up their minds" (the Amplified Bible). The Dutch Bible says it even more clearly: "... gird about the loins of your mind ...".
So what it is the truth, or the belt of truth?
It is all the things that the Word of God says that have been done for you in Christ.
And all you need to do is meditate in them daily, speak them out loud and think on them as you speak.
You will find that the belt of truth is automatically girded about the loins of your mind, the breastplate is in place and you are free to use your shield and sword.
And no sooner!!