Letter 18: This (written from Constantinople in A.D. 381) is the earliest of Jerome's expository letters. In it he explains at length the vision recorded in the sixth chapter of Isaiah, and enlarges upon its mystical meaning.

To Pope Damasus [This letter, written from Constantinople in 381 AD, is Jerome's earliest expository letter. In it he explains at length the vision recorded in Isaiah chapter 6 and elaborates on its mystical meaning. Jerome argues against those who identify "the Lord sitting upon a throne" as God the Father with the seraphim representing the Son and the Holy Spirit. Instead, citing John 12:41, he insists it was Christ whom Isaiah saw. He then proposes that the word "seraphim" -- meaning either "glow" or "beginning of speech" -- represents the Old and New Testaments: "Did not our hearts burn within us while he opened to us the Scriptures?" The letter is notable for Jerome's willingness to cite the original Hebrew alongside various Greek translations, going beyond the Septuagint at a time when even educated Westerners like Augustine did not look past it, and for his independence from Origen's exegesis, which he otherwise greatly admired.]

Letter XVIII. To Pope Damasus. This (written from Constantinople in A.D. 381) is the earliest of Jerome's expository letters. In it he explains at length the vision recorded in the sixth chapter of Isaiah, and enlarges upon its mystical meaning. Some of my predecessors, he writes, make 'the Lord sitting upon a throne' God the Father, and suppose the seraphim to represent the Son and the Holy Spirit. I do not agree with them, for John expressly tells us John 12:41 that it was Christ and not the Father whom the prophet saw. And again, The word seraphim means either 'glow' or 'beginning of speech,' and the two seraphim thus stand for the Old and New Testaments. 'Did not our heart burn within us,' said the disciples, 'while he opened to us the Scriptures?' Luke 24:32 Moreover, the Old Testament is written in Hebrew, and this unquestionably was man's original language. Jerome then speaks of the unity of the sacred books. Whatever, he asserts, we read in the Old Testament we find also in the Gospel; and what we read in the Gospel is deduced from the Old Testament. There is no discord between them, no disagreement. In both Testaments the Trinity is preached. The letter is noticeable for the evidence it affords of the thoroughness of Jerome's studies. Not only does he cite the several Greek versions of Isaiah in support of his argument, but he also reverts to the Hebrew original. So far as the West was concerned he may be said to have discovered this anew. Even educated men like Augustine had ceased to look beyond the LXX., and were more or less aghast at the boldness with which Jerome rejected its time-honored but inaccurate renderings. The letter also shows that independence of judgment which always marked Jerome's work. At the time when he wrote it he was much under the sway of Origen. But great as was his admiration for the master, he was not afraid to discard his exegesis when, as in the case of the seraphim, he believed it to be erroneous.