13 Let him therefore now say certain two things. For greatly to us belong these two things. “For power is of God, and to You, O Lord, is mercy.” Are these the two things, power and mercy? These two evidently: perceive ye the power of God, perceive ye the mercy of God. In these two things are contained nearly all the Scriptures. Because of these two things are the Prophets, because of these two, the Patriarchs, because of these the Law, because of these Himself our Lord Jesus Christ, because of these the Apostles, because of these all the preaching and spreading of the word of God in the Church, because of these two, because of the power of God, and His mercy.
His power fear ye, His mercy love ye. Neither so on His mercy rely, as that His power ye despise: nor so the power fear ye, as that of mercy ye despair. With Him is power, with Him mercy. This man He humbles, and that man He exalts: this man He humbles with power, that man He exalts in mercy. “For if God, willing to show wrath and to prove His power, has in much patience borne with the vessels of wrath, which have been perfected unto perdition” — you have heard of power: inquire for mercy— “and that He might make known,” He says, “His riches unto the vessels of mercy.”
It belongs therefore to His power to condemn unjust men. And to Him who would say, What have you done? “For you, O man, who are you that should make answer to God?” Fear therefore and tremble at His power: but hope for His mercy. The devil is a sort of power; ofttimes however he wishes to hurt, and is not able, because that power is under power. For if the devil could hurt as much as he would; no one of just men would remain, nor could any one of the faithful be on earth. The same through his vessels smites against, as it were, a wall bowed down: but he only smites against, so far as he receives power.
But in order that the wall may not fall, the Lord will support: for He that gives power to the tempter, does Himself to the tempted extend mercy. For according to measure the devil is permitted to tempt. And, “You will give us to drink in tears in a measure.” Do not therefore fear the tempter permitted to do somewhat: for you have a most merciful Saviour. So much he is permitted to tempt as is profitable for you, that you may be exercised, may be proved; in order that by yourself you may be found out, that know not yourself.
For where, or from whence, ought we to be secure, except by this power and mercy of God? After that Apostolic saying, “Faithful is God, that does not suffer you to be tempted above that which you are able.”...Fear not the enemy: so much he does as he has received power to do, fear Him that has the chief power: fear Him, that does as much as He wills, and who does nothing unjustly, and whatever He shall have done, is just. We might suppose something or other to be unjust: inasmuch as God has done it, believe it to be just.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)