Psalms 2:7

David’s reference to the Lord’s decree declaring David "God’s son" goes back to the Davidic Covenant (2 Samuel 7:14). There the Lord described the relationship He would have with David and the kings that would succeed him as that of a father with a son. This communicated to David his legitimate right to rule over Israel. The figure connotes warm affection rather than simply a formal relationship. In the ancient world a king’s son usually succeeded his father on the throne. In Israel, God wanted the kings to regard Him as their Father. From the giving of the Davidic Covenant onward, the term "son," when used of one of the Davidic kings, became a messianic title. It was in this sense that Jesus spoke of Himself as the Son of God. That was a claim to be the Messiah. [Note: See Gerald Cooke, "The Israelite King as Son of God," Zeitschrift für die Alttestamentliche Wissenschaft 73:2 (June 1961):202-25; and Eugene H. Merrill, "The Book of Ruth: Narration and Shared Themes," Bibliotheca Sacra 142:566 (April-June 1985):136-37.]

The "today" in view then is not the day of David’s birth but his coronation, the day he became God’s "son" by becoming king (cf. Matthew 3:17; Mark 1:11; Luke 3:22). Since this psalm deals with a royal coronation, scholars often refer to it as a coronation or enthronement psalm. God begot David in this metaphor not by creating him, though He did that too, but by setting him on the throne of Israel.