St. Augustine the Bishop
No man can serve two masters. And this is further explained: For either he will hate the one, and love the other; or else he will hold to the one, and despise the other. These words we ought carefully to weigh, for the Lord sheweth straightway who be the two masters whom we have choice of; You cannot serve God and Mammon. Mammon is a term which the Hebrews are said to use for riches. It is also a Carthaginian word; for the Punic word for gain is Mammon.
He which serveth Mammon, serveth that evil one who hath perversely chosen to be lord of these earthly things, and is called by the Lord the prince of this world. Of these two masters, either a man will hate the one and love the other (that is God), or he will hold to the one and despise the other. He which serveth Mammon holdeth unto a hard and destroying master, for he is led captive by his lust, and sold slave to the devil, and him loveth no man. Is there any man that loveth the devil? And yet there be many that hold to him.
Therefore, I say unto you, Take no thought for your life, what ye shall eat, or what ye shall drink; nor yet for your body, what ye shall put on: for even though such things are not idle, but needful to be sought after, yet the seeking for things even needful may divide the heart; and thus our intention may be corrupted when we do something as it were merciful; that is, we are to beware lest, when we seem to be seeking another's good, we be but seeking profit to ourselves, under the guise of a benefit to him; and in such wise we seem to ourselves not to sin, because we are seeking things not idle, but needful.
from the Book of Moral Reflections
Some there are who have no regard to their own true life; greedy are they of the things which pass away; but as to the things which are eternal, either they understand them not, or, understanding them, they hold them to be but of little moment; so that they seem insenible, and never know how to take wise advice; and, in forgetfulness of the heavenly possessions which they have lost, they deem themselves (alas, poor wretches!) happy in their possession of the things of the world. They make no endeavour to lift up their eyes to the light of truth for which they were created. No keen desire ever maketh them to cast a longing look toward their eternal fatherland. Rather, forsaking the true end for which they were destined, they fall in love with the exile which they are enduring, rather than with their home, and make merry in their blindness which they are suffering, as though it were glorious daylight.
But, on the other hand, the understandings of the elect, (since they perceive that all things transitory are as nothing,) do go in search after those things for which they were created. And (since nothing outside God doth fully satisfy them) their hearts, even though wearied by the strain of their search, do find rest in the hope for, and in the contemplation of, their Creator. So are they fain to have their citizenship in heaven. And each one of them, although still placed in the world as concerning his body, does yet in heart and mind to it ascend, and there above do continually dwell. All such do bemoan the hardships of the exile which they are enduring, and do rouse themselves by the constant pricking of their love, to look at their fatherland on high. When therefore such an one seeth that he hath, through sin, lost an eternal inheritance, he grieveth; and then he seeketh (and so he findeth) this healthy counsel, to wit, to reckon but lightly the transitory things through which he is passing. And the more he groweth in understanding of the wise course that he hath chosen, (that is, to abandon perishing things,) the more his longing increaseth to attain unto the things which endure.
It is also worthy of notice that they who are given to rash and hasty action are, by the same token, not given to sorrow of heart, but rather are insenible to spiritual things. For they that live without thought, and leave themselves recklessly to the guidance of events, thereby escape the sorrowful weariness which cometh from the effort of thinking. He that ordereth his life by prudent consideration is ever looking carefully round about him before the beginning of any new course. Thus, like a man that, before advancing on an uncertain way, trieth the ground with his foot, so he taketh thought beforehand, lest he come unexpectedly upon some evil thing. On this wise he is ever on the watch; lest panic overtake him when he must do something which requíreth coolness; lest rashness drive him into things which were better put off to another season; lest concupiscence should overcome him in the warfare against his lusts; or lest even good things should undo him by an onslaught of vain glory.