"Understand these words in relation to Christ's becoming man, which took place in time," writes Theophylact, "since the expressions 'today' and 'tomorrow' contain the indication of days.
But the Son is begotten of the Father beyond time."
"The Resurrection of the Lord is the actual divine certification of the sonship of Jesus Christ to God the Father (cf. Rom 1:7).
Through the Lord's Resurrection from the dead, after His abasement and death, God made His Son from eternity to be His Son in time."
"Since they, the Jews, were possessed by fear as those who had killed Christ, and since conscience condemned them, the apostles do not speak to them as Christ-killers.
They present the teaching to them not as an alien good, but as their own. The name of David was greatly desired by the Jews.
Therefore Paul brings him forward, so that by this means at least they may acknowledge Christ. He speaks as though saying: His Son will reign over you, therefore do not reject His authority."
The whole context of Psalm 2 shows this. In its opening words the apostles already saw a prediction of New Testament times (see Acts 4:25-27), namely the Crucifixion of Christ.
Therefore in the final words, in which the Lord addresses His Anointed, one must see a prediction of the Resurrection.
Or, more in keeping with the psalmist's line of thought, one must see the full acknowledgment by God the Father of Christ's filial dignity through His Resurrection from the dead.
It is, as it were, the birth of the Messiah through the Resurrection. The Resurrection is the actual divine certification of the sonship of Jesus Christ to God the Father (see Rom 1:4).
In the Epistle to the Hebrews, the apostle cites this saying twice (Heb 1:5; Heb 3:5). He grounds in it the kingly and high-priestly dignity of Jesus Christ.
Both began only in the Resurrection.
The final words of verse 34 are taken from the book of the prophet Isaiah (Is 55:3). The apostle cites them according to the Septuagint, with a slight alteration.
In these words the prophet expresses the promise of the coming of the messianic times. He speaks of them directly in the first three verses of chapter 55.
To certify this, instead of his usual forms of assurance, the prophet sets forth the unfailing fulfillment of the mercies promised to David. By these are meant all the mercies of messianic salvation.