Rightly is that called fortitude, when a man conquers himself, restrains his anger, yields and gives way to no allurements, is not put out by misfortunes, nor gets elated by good success, and does not get carried away by every varying change as by some chance wind.
[…] Fortitude of the mind can be regarded in two ways. First, as it counts all externals as very unimportant, and looks on them as rather superfluous and to be despised than to be sought after.
Secondly, as it strives after those things which are the highest, and all things in which one can see anything moral (or as the Greeks call it, πρέπον), with all the powers of the mind.
For what can be more noble than to train your mind so as not to place a high value on riches and pleasures and honours, nor to waste all your care on these?
When your mind is thus disposed, you must consider how all that is virtuous and seemly must be placed before everything else.
And you must so fix your mind upon that, that if aught happens which may break your spirit, whether loss of property, or the reception of fewer honours, or the disparagement of unbelievers, you may not feel it, as though you were above such things; nay, so that even dangers which menace your safety, if undertaken at the call of justice, may not trouble you.
This is the true fortitude which Christ’s warrior has, who receives not the crown unless he strives lawfully (2 Tim. 2:5). Or does that call to fortitude seem to you but a poor one: “Tribulation worketh patience, and patience, experience, and experience, hope”? (Rom. 5:3-4). See how many a contest there is, yet but one crown!
[…] Think how St Paul teaches those who enter upon their duties in the Church that they ought to have contempt for all earthly things: “If, then, ye be dead with Christ from the elements of this world, why do ye act as though living in the world?” (Col. 2:20-21).
And further: “If ye then be risen with Christ, seek those things which are above, not those things which are on the earth” (Col. 3:1-2). And again: “Mortify, therefore, your members which are on the earth” (Col. 3:5).
This, indeed, is meant for all the faithful. But you, especially, my son, he urges to despise riches and to avoid profane and old wives fables—allowing nothing but this: “Exercise thyself unto godliness, for bodily exercise profiteth a little, but godliness is profitable unto all things” (1 Tim. 4:8).
Ambrose of Milan (c. 337-397): On the Duties of the Clergy, book 1, chapter 36, 181-184.