45 Did the Jewish children fear the flames blazing up with the fuel cast upon them in the fiery furnace at Babylon? Did the terror of that terrible fire prevail over their nature, conceived though it was like ours? Did they feel pain, when the flames surrounded them? Perhaps, however, you may say they felt no pain, because they were not burnt: the flames were deprived of their burning nature. To be sure it is natural to the body to fear burning, and to be burnt by fire.
But through the spirit of faith their earthly bodies (that is, bodies which had their origin according to the principles of natural birth) could neither be burnt nor made afraid. What, therefore, in the case of men was a violation of the order of nature, produced by faith in God, cannot be judged in God's case natural, but as an activity of the Spirit commencing with His earthly origin. The children were bound in the midst of the fire; they had no fear as they mounted the blazing pile: they felt not the flame as they prayed: though in the midst of the furnace, they could not be burnt.
Both the fire and their bodies lost their proper natures; the one did not burn, the others were not burnt. Yet in all other respects, both fire and bodies retained their natures: for the bystanders were consumed, and the ministers of punishment were themselves punished. Impious heretic, you will have it that Christ suffered pain from the piercing of the nails, that He felt the bitterness of the wound, when they were driven through His hands: why, pray, did not the children fear the flames?
Why did they suffer no pain? What was the nature in their bodies, which overcame that of fire? In the zeal of their faith and the glory of a blessed martyrdom they forgot to fear the terrible; should Christ be sorrowful from fear of the cross, Christ, Who even if He had been conceived with our sinful origin, would have been still God upon the cross, Who was to judge the world and reign for ever and ever? Could He forget such a reward, and tremble with the anxiety of dishonourable fear?
Source: On the Trinity (New Advent)