Extracts and comments from Revelation Chap. 9 V.17
And thus I saw the horses in the vision, and them that sat on them, having breastplates of (a)fire, and of (b)jacinth, and (c)brimstone: and the heads of the horses were as the heads of (d)lions; and out of the mouths issued (d)fire and smoke and (c)brimstone.
(c) "Brimstone"; 1) sulphurous smoke / fire. 2) Stone burning. It is of interest that all the above describe the "breastplate". Scripture's first mention of it is Ex. 25:7, just after Moses 40 days and nights with Jehovah on top of Mount Sinai, and it is placed over the heart (on the breast) of the priest. It was covered with 12 precious stones, one for each tribe of Israel, and folded like a pouch, to hold the Urim and Thummim, (for which was derived the will of God).
In later description, the great Apostle Paul shows the need for "the breastplate of faith and love", (1 Thess. 5:8). It is not therefore surprising that Satan's agents should produce in their terrors an anti- thesis.
The demonstration of Jehovah in the breastplate is love for Israel, (later also for His Body), His will to Israel (Urim and Thummim), and always it covered the heart of the anointed high priest (symbolic of the future Great High Priest – Christ).
In the case of the Acts period Christians, no priest was needed or allowed, which caused great consternation among the Jewish Priests and hierarchy. Every individual stood before Christ alone, which contrary to current teaching in many groups is still the case. Therefore the breastplate is of "faith and love". In the Satanic charge, the breastplate is of pain, torture and death, instead of the life and hope of Christ.