436. And they had breastplates like breastplates of iron. (9:9) This symbolically means that their arguments based on fallacies, with which they battled and prevailed, appeared to them too strong to be refuted.

Breastplates symbolize protections, because they protect the breast. Here they symbolize protections of falsities, which are concocted by arguments based on fallacies, which people use to defend a false proposition. For from a false proposition nothing but falsities can flow. If truths are advanced, they are regarded only externally or superficially, thus also sensually, and so are falsified, becoming then fallacies in the people who entertain them.

Breastplates have this symbolism because battles in the Word symbolize spiritual battles, and weapons of war therefore symbolize various defenses connected with such a battle - as in Jeremiah,

Harness the horses, and mount up, you horsemen! And station yourselves in your helmets, polish the spears, put on the cuirass. (Jeremiah 46:4)

In Isaiah,

He put on righteousness as a breastplate, and a helmet of salvation on His head. (Isaiah 59:17)

In the book of Psalms,

...under His wings you shall be confident; His truth shall be your shield and buckler. (Psalms 91:4)

And so also elsewhere, as Ezekiel 23:24; 38:4; 39:9, Nahum 2:3, Psalm. 5:12; 35:2-3.

Their breastplates being breastplates of iron means, symbolically, that their arguments seemed to them too strong to be refuted; for owing to its hardness iron symbolizes strength.

In Psalm 40:27, this signifies horrible evils from which the Lord saves us. (The Inner Meaning of the Prophets and Psalms 290)

In Isaiah 29:16 and 64:8, this signifies the good of which the mind or the man himself of the church is formed and from which proceed his reformation and regeneration. (Arcana Coelestia 1300)

In Daniel 2:33, this signifies goods from lower natural things. (Arcana Coelestia 2162[3])

In Nahum 3:14, this signifies falsities. (Arcana Coelestia 1296)

In John 9:6, this signifies reformation through truth from the literal sense of the Word. (Apocalypse Explained 239[19])

(References: Apocalypse Explained 355)