1 Kings 20:11

20:11 The king of Israel replied, “Tell him the one who puts on his battle gear should not boast like one who is taking it off.” ¹ 

Ephesians 6:14

6:14 Stand firm therefore, by fastening ²  the belt of truth around your waist, ³  by putting on the breastplate of righteousness,

[20:11]  ¹ sn The point of the saying is that someone who is still preparing for a battle should not boast as if he has already won the battle. A modern parallel would be, “Don’t count your chickens before they’re hatched.”

[6:14]  ² sn The four participles fastening… putting on…fitting…taking up… indicate the means by which believers can take their stand against the devil and his schemes. The imperative take in v. 17 communicates another means by which to accomplish the standing, i.e., by the word of God.

[6:14]  ³ tn Grk “girding your waist with truth.” In this entire section the author is painting a metaphor for his readers based on the attire of a Roman soldier prepared for battle and its similarity to the Christian prepared to do battle against spiritually evil forces. Behind the expression “with truth” is probably the genitive idea “belt of truth.” Since this is an appositional genitive (i.e., belt which is truth), the author simply left unsaid the idea of the belt and mentioned only his real focus, namely, the truth. (The analogy would have been completely understandable to his 1st century readers.) The idea of the belt is supplied in the translation to clarify the sense in English.