Cyprian had visited Jerome at Bethlehem and had asked him to write an exposition of Psalm XC. in simple language such as might be readily understood. With this request Jerome now complies, giving a very full account of the psalm, verse by verse, and bringing the treasures of his learning and especially his knowledge of Hebrew to bear upon it. He asserts its Mosaic authorship but is careful to add that "the man of God" may have spoken not for himself but in the name of the Jewish people. He speaks of the five books into which the psalter is divisible and says that it is a mistake to ascribe all the psalms to David. An allusion to the doctrine of Pelagius shows that the letter must belong to Jerome's last years, and Vallarsi is probably right in assigning it to a.d.418.