“Real glory springs from the silent conquest of ourselves; and without that the conqueror is naught but the first slave.” — James Thomson
(Overcoming our biases and preconceptions, and or other faults??? You know me, folks. Poetry is not my strong suit. I’m guessing the line is from the James Thomson above, but I guess it could be from the slightly later one. Does anyone know which one this line belongs to? The quotation – including the author’s name – uses every letter but K, X and Z, which I think is pretty cool! Be well and do good, friends.) — YUR
Unclerave's Wordy Weblog 
This one was a little challenging, perhaps because it was (as you noted) nearly a pangram. I quickly spotted “THAT THE” and inferred AND after the comma – giving me 6 letters that supply nearly 50% of written English – but still had to ponder the unusual (for Cryptoquotes) words GLORY, SPRINGS, CONQUEST, NAUGHT and SLAVE. Also, I have no idea about the attribution.