16 Do you then perceive that if a man does not injure himself, no one else will be able to harm him? Behold at least youthfulness, and captivity and destitution, and removal into a foreign land, and loneliness, and dearth of protectors, and a stern command, and great fear of death assailing the mind of the eunuch, and poverty, and feebleness of numbers, and dwelling in the midst of barbarians, and having enemies for masters, and surrender into the hands of the king himself, and separation from all their kindred, and removal from priests and prophets, and from all others who cared for them, and the cessation of drink offerings and sacrifices, and loss of the temple and psalmody, and yet none of these things harmed them; but they had more renown then than when they enjoyed these things in their native land. And after they had accomplished this task first and had wreathed their brows with the glorious garland of victory, and had kept the law even in a foreign land, and trampled under foot the tyrant's command, and overcome fear of the avenger, and yet received no harm from any quarter, as if they had been quietly living at home and enjoying the benefit of all those things which I mentioned, after they had thus fearlessly accomplished their work they were again summoned to other contests. And again they were the same men; and they were subjected to a more severe trial than the former one, and a furnace was kindled, and they were confronted by the barbarian army in company with the king: and the whole Persian force was set in motion and everything was devised which tended to put deceit or constraint upon them: various kinds of music, and various forms of punishment, and threats, and what they saw on every side of them was alarming, and the words which they heard were more alarming than what they saw; nevertheless inasmuch as they did not betray themselves, but made the most of their own strength, they never sustained any kind of damage: but even won for themselves more glorious crowns of victory than before. For Nabuchadonosor bound them and cast them into the furnace, yet he burnt them not, but rather benefited them, and rendered them more illustrious. And although they were deprived of temple (for I will repeat my former remarks) and altar, and fatherland, and priests and prophets, although they were in a foreign and barbarous country, in the very midst of the furnace, surrounded by all that mighty host, the king himself who wrought this looking on, they set up a glorious trophy, and won a notable victory, having sung that admirable and extraordinary hymn which from that day to this has been sung throughout the world and will continue to be sung to future generations.
Thus then when a man does not injure himself, he cannot possibly be hurt by another: for I will not cease harping constantly upon this saying. For if captivity, and bondage, and loneliness and loss of country and all kindred and death, and burning, and a great army and a savage tyrant could not do any damage to the innate virtue of the three children captives, bondmen, strangers though they were in a foreign land, but the enemy's assault became to them rather the occasion of greater confidence: what shall be able to harm the temperate man? There is nothing, even should he have the whole world in arms against him. But, some one may say, in their case God stood beside them, and plucked them out of the flame. Certainly He did; and if you will play your part to the best of your power, the help which God supplies will assuredly follow.
Source: No One Can Harm the Man Who Does Not Injure Himself (New Advent)