Which refers to His resurrection, "today I have begotten You" is used in reference to "He has raised up Jesus"; Christ, raised and glorified, ascended and seated at the right hand of the Father fulfills the promise, "and the Lord God will give to him the throne of his ancestor David. He will reign over the house of Jacob forever, and of his kingdom there will be no end." (Luke 1:32-33).
But when we speak of Jesus as the Son of God, begotten of the Father, we speak of his eternal generation from the Father, as we confess in the Nicene Creed,
"We believe in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all ages. God from God, Light from Light, true God from true God, begotten, not made, of the same Being with the Father; through Him all things were made."
Even as Scripture says that He is the Logos who was in the beginning with God and is God (John 1:1), the only-begotten who is at the bosom of the Father (John 1:18).
-CryptoLutheran