Revelation 9:9 They had breastplates of iron

⁹ And they had breastplates, as it were breastplates of iron; and the sound of their wings was as the sound of chariots of many horses running to battle. -Revelation 9:9 King James Version (KJV)

Apocalypse Revealed, by Emanuel Swedenborg, [1766], tr. by John Whitehead [1912], at sacred-texts.com

436.

Verse 9. And they had breastplates, as breastplates of iron, signifies arguments from fallacies, [a mistaken belief, especially one based on unsound arguments] by which they fight and prevail, which appeared to them so powerful that they could not be refuted. By "breastplates" are signified defenses, because they protect the breast; here, defenses of falsities, which are effected by arguments from fallacies, by which a false principle is defended. For from a false principle nothing but falsities can flow. If truths are advanced, they are only viewed outwardly or superficially... and are consequently falsified, and, with such persons, become fallacies. The reason why "breastplates" have this signification, is because "wars," in the Word, signify spiritual wars, and thence the "arms of war" signify the various things relating to such wars... "Their breastplates were as of iron," signifies that their arguments seemed to them so strong that they could not be refuted; for "iron" from its hardness, signifies what is strong.

437.

And the voice of their wings was as the voice of chariots of many horses running to war, signifies their reasonings as if they were from truths of doctrine from the Word fully understood, for which they must ardently fight. "The voice of wings" signifies reasonings, because "to fly" signifies to perceive and instruct (n. 245, 415); "chariots" signify doctrinals, as will be seen presently; "horses" signify the understanding of the Word (n. 298); and "many horses," what is plenary; [Crusaders were offered a plenary (unconditional) indulgence (in the Roman Catholic Church a grant by the Pope of remission of the temporal punishment in purgatory still due for sins) by the Pope] that "running to war" signifies ardour [great enthusiasm or passion] for fighting, is evident...