I will declare the decree: the LORD hath said unto me, Thou [art] my Son; this day have I begotten thee.
Ver. 7. I will declare the decree] That irrevocable decree of the Father, for the setting up of his Son's sceptre contra gentes, point blank opposite to that decree of theirs, Psalms 2:3. This ordinance or decree of his Christ is still declaring in his Church by the ministers of the gospel, whose office it is to set forth Christ to the world in all his offices and efficacies, and to bring as many as may be to the obedience of the faith.
Thou art my Son] David was so by adoption and acceptation, Psalms 89:26,27; but Christ, 1. By eternal generation, Pro 8:22-23 Hebrews 1:5 Hebrews 1:2. By hypostatical union, and so God had one only Son (as Abraham had his Isaac), though otherwise he were the Father of many nations.
This day have I begotten thee] Understand it either in the day of eternity, or else of the fulness of time wherein God brought his first begotten Son into the world, and afterwards mightily declared him to be the Son of God by the resurrection from the dead, Rom 1:3 Acts 13:33, whence he is called the first begotten of the dead, Colossians 1:18 Revelation 1:5 .