5 “Take therefore no thought for the morrow: for sufficient unto the day is the evil thereof:” that is to say, the affliction, and the bruising thereof. Is it not enough for you, to eat your bread in the sweat of your face? Why add the further affliction that comes of anxiety, when you are on the point to be delivered henceforth even from the former toils?
By “evil” here He means, not wickedness, far from it, but affliction, and trouble, and calamities; much as in another place also He says, “Is there evil in a city, which the Lord has not done?” nor any thing like these, but the scourges which are borne from above. And again, “I,” says He, “make peace, and create evils:” For neither in this place does He speak of wickedness, but of famines, and pestilences, things accounted evil by most men: the generality being wont to call these things evil. Thus, for example, the priests and prophets of those five lordships, when having yoked the cattle to the ark, they let them go without their calves, gave the name of “evil” to those heaven-sent plagues, and the dismay and anguish which thereby sprang up within them.
This then is His meaning here also, when He says, “sufficient unto the day is the evil thereof.” For nothing so pains the soul, as carefulness and anxiety. Thus did Paul also, when urging to celibacy, give counsel, saying, “I would have you without carefulness.”
But when He says, “the morrow shall take thought for itself,” He says it not, as though the day took thought for these things, but forasmuch as He had to speak to a people somewhat imperfect, willing to make what He says more expressive, He personifies the time, speaking unto them according to the custom of the generality.
And here indeed He advises, but as He proceeds, He even makes it a law, saying, “provide neither gold nor silver, nor scrip for your journey.” Thus, having shown it all forth in His actions, then after that He introduces the verbal enactment of it more determinately, the precept too having then become more easy of acceptance, confirmed as it had been previously by His own actions. Where then did He confirm it by His actions? Hear Him saying, “The Son of Man has not where to lay His head.” Neither is He satisfied with this only, but in His disciples also He exhibits His full proof of these things, by fashioning them too in like manner, yet not suffering them to be in want of anything.
But mark His tender care also, how He surpasses the affection of any father. Thus, “This I command,” says He, “for nothing else, but that I may deliver you from superfluous anxieties. For even if today you have taken thought for tomorrow, you will also have to take thought again tomorrow. Why then what is over and above? Why force the day to receive more than the distress which is allotted to it, and together with its own troubles add to it also the burden of the following day; and this, when there is no chance of your lightening the other by the addition so taking place, but you are merely to exhibit yourself as coveting superfluous troubles?” Thus, that He may reprove them the more, He does all but give life to the very time, and brings it in as one injured, and exclaiming against them for their causeless despite. Why, you have received the day, to care for the things thereof. Wherefore then add unto it the things of the other day also? Hath it not then burden enough in its own anxiety? Why now, I pray, do you make it yet heavier? Now when the Lawgiver says these things, and He that is to pass judgment on us, consider the hopes that He suggests to us, how good they are; He Himself testifying, that this life is wretched and wearisome, so that the anxiety even of the one day is enough to hurt and afflict us.
Source: Homilies on the Gospel of St. Matthew (New Advent)