2:7 my Son. In the third three-verse stanza, the Son speaks, noting that He is, indeed, the very Son of God!
2:7 declare the decree. The Father gave the decree, the Son declared it (note John 1:18).
2:7 I begotten thee. There are several senses in which Christ is the only begotten Son of God, but the emphasis here is on His resurrection from the dead, as evident from the quotation of this verse in Acts 13:33. He was “declared to be the Son of God with power,...by the resurrection from the dead” (Romans 1:4). He was also called the “firstborn from the dead” (Colossians 1:18) and the “first begotten of the dead” (Revelation 1:5). Note also Hebrews 5:5.
2:8 thine inheritance. The Son is declared to be the “heir of all things” (Hebrews 1:2), and His followers are “joint-heirs” with Him (Romans 8:16, 17).