4 “Say ye to God, How to be feared are Your works!”. Wherefore to be feared and not to be loved? Hear another voice of a Psalm: “Serve the Lord in fear, and exult unto Him with trembling.” What means this? Hear the voice of the Apostle: “With fear,” he says, “and trembling, work out your own salvation.” Wherefore with fear and trembling? He has subjoined the reason: “for God it is that works in you both to will and to work according to good will.” If therefore God works in you, by the Grace of God you work well, not by your strength.
Therefore if you rejoice, fear also: lest perchance that which was given to a humble man be taken away from a proud one....Brethren, if against the Jews of old, cut off from the root of the Patriarchs, we ought not to exalt ourselves, but rather to fear and say to God, “How to be feared are Your works:” how much less ought we not to exalt ourselves against the fresh wounds of the cutting off! Before there had been cut off Jews, graffed in Gentiles; from the very graft there have been cut off heretics; but neither against them ought we to exalt ourselves; lest perchance he deserve to be cut off, that delights to revile them that are cut off.
My brethren, a bishop's voice, however unworthy, has sounded to you: we pray you to beware, whosoever you are in the Church, do not revile them that are not within; but pray ye rather, that they too may be within. For God is able again to graft them in. Of the very Jews the Apostle said this, and it was done in their case. The Lord rose again, and many believed: they perceived not when they crucified, nevertheless afterwards they believed in Him, and there was forgiven them so great a transgression.
The shedding of the Lord's blood was forgiven the manslayers, not to say, God-slayers: “for if they had known, the Lord of glory they never would have crucified.” Now to the manslayers has been forgiven the shedding of the blood of Him innocent: and that same blood which through madness they shed, through grace they have drunk....O fullness of Gentiles, say thou to God, “How to be feared are Your works!” and so rejoice thou as that you may fear, be not exalted above the branches cut off.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)