19 Attend, brethren; it is for sale. What I have is for sale, says God unto you, buy it. What has He for sale? I have rest for sale; buy it by your toil. Attend, that we may be in Christ's name brave Christians: the remainder of the Psalm is but a little, let us not be weary. For how can he be strong in doing, who fails in hearing? The Lord will help us to expound unto you the remainder. Attend then: God has, as it were, proclaimed the kingdom of heaven for sale. You say unto Him, What is its value? The price is toil: if He were to say, its price is gold, it would not suffice to say this only, but you would seek to know how much gold; for there is a mass of gold, and half an ounce, and a pound, and the like. He said “price,” that you might not be at pains to inquire, how long you should find it. The price of the commodity is toil: how much toil is it? Now seek how much you should toil for it. You are not as yet told how great that toil is doomed to be, or how much toil is required of you: God says this unto you, I show you how great that rest will be; do thou judge with what measure of toil it should be bought.
20....He promised rest: suffer trouble. He threatens eternal fire; despise temporal pains: and while Christ does watch, let your heart be calmed, that you also may reach the harbour. For He would not fail to prepare a harbour, who provided a vessel. “Have You anything to do with the stool of iniquity, Thou who makest sorrow in learning?” He tries us with the wicked, and by their persecution He teaches us. By means of the malice of the wicked the good is scourged, through the slave the son is chastened: thus is learning taught by sorrow. What God allows them power to do, that do wicked men, whom He spares for a season, do.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)