8 The matter next to be introduced, moreover, is one requiring, in order to its right apprehension and contemplation, a reader of the greatest attention and carefulness. For it has been acutely observed that Matthew, who had proposed to himself the task of commending the kingly character in Christ, named, exclusive of Christ Himself, forty men in the series of generations. Now this number denotes the period in which, in this age and on this earth, it behooves us to be ruled by Christ in accordance with that painful discipline whereby “God scourges,” as it is written, “every son that He receives;” and of which also an apostle says that “we must through much tribulation enter into the kingdom of God.” This discipline is also signified by that rod of iron, concerning which we read this statement in a Psalm: “You shall rule them with a rod of iron;” which words occur after the saying, “Yet I am set king by Him upon His holy hill of Zion!” For the good, too, are ruled with a rod of iron, as it is said of them: “The time has come that judgment should begin at the house of God; and if it first begin at us, what shall the end be to them that obey not the gospel of God? And if the righteous scarcely be saved, where shall the ungodly and the sinner appear?” To the same persons the sentence that follows also applies: “You shall dash them in pieces like a potter's vessel.” For the good, indeed, are ruled by this discipline, while the wicked are crushed by it. And these two different classes of persons are mentioned here as if they were the same, on account of the identity of the signs employed in reference to the wicked in common with the good.
Source: The Harmony of the Gospels (New Advent)