Lessons 7-9 from the Divine Office of St. Anthony of Padua, Confessor and Doctor of the Church: Matt 5:13-19; Book 1, Chapter 6 on the Sermon on the Mount by St. Augustine the Bishop.
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At that time Jesus said to His disciples: You are the salt of the earth; but if the salt loses its strength, what shall it be salted with? It is no longer of any use but to be thrown out and trodden underfoot by men. You are the light of the world. A city set on a mountain cannot be hidden. Neither do men light a lamp and put it under the measure, but upon the lampstand, so as to give light to all in the house. Even so let your light shine before men, in order that they may see your good works and give glory to your Father in heaven. Do not think that I have come to destroy the Law or the Prophets. I have not come to destroy, but to fulfill. For amen I say to you, till heaven and earth pass away, not one jot or one tittle shall be lost from the Law till all things have been accomplished. Therefore whoever does away with one of these least commandments, and so teaches men, shall be called least in the kingdom of heaven; but whoever carries them out and teaches them, he shall be called great in the kingdom of heaven.
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The Lord would have us understand how that men do lose their power of savouring others with righteousness when they are willing to place their eternal welfare in jeopardy for the sake of any temporal advantage, like as attainment of ease or luxury, or escape from suffering or toil. For that which is eternal, unlike things of this world, can neither be bestowed by men, nor by them taken away. Hence, when he asketh: If the salt have lost his savour, wherewith shall it be salted? he would have us understand the question to be: If ye, by whom mankind is preserved from corruption, be willing to lose the kingdom of heaven so as to escape trials or persecutions in this world, who is there to preserve you from corruption, seeing ye are they that God hath chosen to preserve all others from corruption?
Those that should be the salt of the earth, but have lost their savour, are thenceforth good for nothing, but to be cast out, and to be trodden under foot of men. But no one that suffereth persecution is truly said to be trodden under foot of men. Rather, that one is truly trodden under foot of men who through fear of persecution hath lost the savour of righteousness. For no one can be trodden upon, unless he be beneath him which treadeth upon him. And certainly no one who hath his heart in heaven, no matter how grievously he doth suffer in his body on earth, is rightly said to be beneath anyone who misuseth him.
Ye are the light of the world. And we are to understand the word World in the same sense as the word Earth when he spoke above of the salt of the earth, that is, not that earth whereupon we walk with our bodily feet, but the men which dwell upon the earth; in other words, sinners, for the sweetening and correction of whose corruption, the Lord hath sent his Apostles, as it were, as so much salt. And so by the world we are to understand, not the heaven and the earth, but the men who are in the world and love the world, for the enlightening of whom the Apostles have been sent. A city that is set on a hill cannot be hid: that is, what is founded upon the heights of righteousness, whereof the mountain upon which the Lord gave this discourse was itself a figure, is magnificent in the eyes of all men.