3 Then, because the saying seemed to be vehement, see how He softens it by what follows, and sets down rewards surpassing our toils; and not rewards only, but also the penalties of vice: nay, on these last He dwells more than on those, since not so much His bestowing blessings, as His threat of severities, is wont to bring ordinary men to their senses. See at least how He both begins here from this, and ends in this.
“For whosoever will save his life shall lose it,” says He, “but whosoever shall lose his life for my sake, shall find it. For what is a man profited, if he should gain the whole world, and lose his own soul? Or what shall a man give in exchange for his soul?”
Now what He says is like this: “not as unsparing towards you, but rather as exceedingly sparing you, I enjoin these things. For he who spares his child, ruins it; but he who spares it not, preserves.” To which effect also a certain wise man said, “If you beat your son with a rod, he shall not die, but you shall deliver his soul from death.” And again, “He that refreshes his son, shall bind up his wounds.”
This takes place in the camp also. For if the general, sparing the soldiers, commands them to remain within the place always, he will destroy with them the inhabitants too.
“In order then that this may not happen in your case also,” says He, “ye must be arrayed against continual death. For now too a grievous war is about to be kindled. Sit not therefore within, but go forth and fight; and should you fall in your post, then have you obtained life.” For if in the visible wars he that in his post meets slaughter, is both more distinguished than the rest, and more invincible, and more formidable to the enemy; although we know that after death the king, in behalf of whom he takes his station, is not able to raise him up again: much more in these wars, when there are such hopes of resurrection besides, will he who exposes his own life unto death, find it; in one sense, because he will not be quickly taken; in a second, because even though he fall, God will lead his life on to a higher life.
Source: Homilies on the Gospel of St. Matthew (New Advent)