Evangelus had sent Jerome an anonymous treatise in which Melchisedek was identified with the Holy Ghost, and had asked him what he thought of the theory. Jerome in his reply repudiates the idea as absurd and insists that Melchisedek was a real man, possibly, as the Jews said, Shem the eldest son of Noah. The date of the letter is 398 a.d.
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 6 (Schaff and Wace, 1893), public domain (Public domain).