7 He next follows out the history well known in the truth of the holy Scriptures. “When they were in small numbers, very few, and they strangers in the land”; that is, in the land of Canaan....But some copies have the words “very few, and they strangers,” in the accusative case, the translator having turned the Greek phrase too literally into Latin. If we were to render the whole clause in this way, we must say, “that they were very few, and they strangers;” but the phrase, “while they were,” is the meaning of the Greek; and the verb, “to be,” takes not an accusative, but a nominative after it.
8. “What time as they went from one nation to another, from one kingdom to another people”. This is a repetition of what he had said, “from one nation to another.” “He suffered no man to do them harm: but reproved even kings for their sakes”. “Touch not,” He said, “Mine anointed, and do My prophets no harm”. He declares the words of God chiding or reproving kings, that they might not harm the holy fathers, while they were small in number, very few, and they strangers in the land of Canaan. Although these words be not read in the books of that history, yet they are to be understood as either secretly spoken, as God speaks in the hearts of men by unseen and true visions, or even as announced through an Angel. For both the king of Gerar and the king of the Egyptians were warned from Heaven not to harm Abraham, and another king not to harm Isaac, and others not to harm Jacob; while they were very few, and strangers, before he went over into Egypt to sojourn with his sons: which is understood to be herein mentioned. But since it occurred to ask, before they passed over and multiplied in Egypt, how so few in number, and those strangers in a foreign land, could maintain themselves: he next adds, “He suffered no man to do them wrong,” etc.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)