20 “You have led home Your people like sheep in the hand of Moses and of Aaron”. Why He has added this is somewhat difficult to discover....They banished Christ; sick as they were, they would not have Him for their Saviour; but He began to be among the Gentiles, and among all nations, among many peoples. Nevertheless, a remnant of that people has been saved. The ungrateful multitude has remained without, even the halting breadth of Jacob's thigh. For the breadth of the thigh is understood of the multitude of lineage, and among the greater part of the Israelites a certain multitude became vain and foolish, so as not to know the steps of Christ on the waters. “You have led home Your people like sheep,” and they have not known You. Though You have done such great benefits unto them, hast divided sea, hast made them pass over dry land between waters, hast drowned in the waves pursuing enemies, in the desert hast rained manna for their hunger, leading them home “by the hand of Moses and Aaron:” still they thrust You from them, so that in the sea was Your Way, and Your steps they knew not.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)