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Saint Augustine of Hippo, Confessions , III.iv:
 
In the ordinary course of study I came upon a certain book of Cicero's....This particular book of his contains an exhortation to philosophy and was called Hortensius .  Now it was this book which quite definitely changed my whole attitude and turned my prayers toward thee, O Lord, and gave me new hope and new desires.  Suddenly every vain hope became worthless to me, and with an incredible warmth of heart I yearned for an immortality of wisdom....
Julius Caesar (as told by Pliny the Elder, Natural History VII.117):
 
[Cicero was] winner of a greater laurel wreath than any gained from a triumph, inasmuch as it is greater to have advanced the frontiers of the Roman genius than those of the Roman empire.
Edward Gibbon, History of the Rise and Decline of the Roman Empire, ch. 2:
 
...the little town of Arpinum claimed the double honour of producing Marius and Cicero...; and the latter, after saving his country from the designs of Catiline, enabled her to contend with Athens for the palm of eloquence.
Thomas Jefferson, letter to John Adams , 5 July 1815:
 
Altho' Cicero did not wield the dense logic of Demosthenes, yet he was able, learned, laborious, practised in the business of the world, and honest. He could not be the dupe of mere style, of which he was himself the first master in the world.
Francesco Petrarch, Letter to Marcus Tullius Cicero:
 
O great father of Roman eloquence! not I alone but all who deck themselves with the flowers of Latin speech render thanks unto you. It is from your well-springs that we draw the streams that water our meads. You, we freely acknowledge, are the leader who marshals us; yours are the words of encouragement that sustain us; yours is the light that illumines the path before us. In a word, it is under your auspices that we have attained to such little skill in this art of writing as we may possess.... 
Pliny the Elder, Natural History VII.116-17:
 
There are countless Roman examples, if one were disposed to pursue them, since this single race has produced more distinguished men than other countries in every field of activity.  But what excuse could I have for not mentioning you, Marcus Tullius?...Catiline fled from your genius.  You proscribed Marc Antony.  Hail first person to be called "Father of your Country," the first civilian to win a triumph and a laurel wreath for his speeches, parent of eloquence and of Latium's literature.
Quintillian:
 
Cicero was the name not of a man, but of eloquence itself.
L. Annaeus Seneca, De Brevitate Vitae 5:
 
When Marcus Cicero was cast among men like Catiline and Clodius and Pompey and Crassus--some of them undisguised enemies and some doubtful friends--when he was tossed about in the storm that struck the state, he tried to hold it steady as it went to its doom; but at last he was swept away....[H]ow often does he curse that consulship, which he had praised without ceasing though not without good reason!
Medieval opinion of Cicero (from Cicero's Rhetoric In The Middle Ages):
 
As far as allusions to Cicero's rhetoric, there are indications that he was considered to be top notch, untouchable. Murphy states that during the Middle Ages, he (Cicero) is viewed as the unquestioned Master of Eloquence (or magister eloquential) (334).  Numerous teachers, encyclopedists, and royalty--from Saint Augustine to Dante--raved about Cicero's rhetoric. Indeed, there is hardly a major medieval writer who does not mention Cicero whenever there is occasion to speak of discourse (334). Even those who wrote sermon and letter-writing manuals used Cicero's knowledge to enhance their works (335). 

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