Say you’re checking out at the supermarket and as the grocer goes to weigh your produce on the electronic scales you notice that there is dust on the scales. Do you yell out, “Hold on! Clean the scales off first, I don’t pay for dust.” Of course you don’t, because dust is counted as nothing. In Isaiah 40:15 we learn that all the nations arrayed in all their pomp, splendor, vehemence, and power are accounted by God as dust on the scales and we also see that truth here.
Here we see the scepter of the Son crushing all enemies, but do not miss grace for the fury. God the Son has eternally ruled (Colossians 1:16-17). This text does not teach against the Son’s eternal rule, but it does teach something in addition to it. Here we have a King not just over us, but for us.
The this psalm deals with the Messiah’s session (v.1). Jesus’ session is the important, but too often neglected doctrine concerning Jesus’ being seated on His throne at the right hand of the Father. This specific session follows Christ’s priestly action for us (Hebrews 1:3; Ephesians 1:20-23). This is the main connection I think you are meant to see between God’s oracle spoken to the Messiah as King (v.1), and His oath spoken to the Messiah as Priest (v.4). Jesus priestly work and kingly work are interrelated.
Christ, at his weakest moment in human flesh, acting as our High Priest, was also a King conquering our greatest foes – Satan, death, and sin. He was the meekest Lamb and the fiercest Lion in the same act. So Jesus’ priestly work is also king work, and his Kingly rule is also priestly action.
When Jesus comes to quell all rebellion in the day of His power, it will also be an outworking of His priestly action for us. Because of Christ, all that God is, He is for us. You may only think of God as being for you in His grace, mercy, kindness, love, faithfulness, goodness, and patience, but in Christ His sovereignty, power, justice, righteousness, judgment, and yes, even His wrath are also for us – total God, totally for you. In Christ His enemies are now ours. Even God’s wrath against His enemies now works to our salvation in Christ.
So, to put succinctly, how can God’s wrath be for our good? In Jesus – because Jesus bore the wrath of God as our substitute, when He returns, His anger unleashed upon His enemies will also mean the end of those who oppose our greatest joy. His wrath will not burn against us, but for us.