Today I have begotten you
When the begetting of the Son occurs — eternally, at the resurrection, or at the baptism.
Hebrews 1:5-14
5For to which of the angels did he say at any time, “You are my Son. Today I have become your father?” and again, “I will be to him a Father, and he will be to me a Son?”
6When he again brings in the firstborn into the world he says, “Let all the angels of God worship him.”
7Of the angels he says, “He makes his angels winds, and his servants a flame of fire.”
8But of the Son he says, “Your throne, O God, is forever and ever. The scepter of uprightness is the scepter of your Kingdom.
9You have loved righteousness and hated iniquity; therefore God, your God, has anointed you with the oil of gladness above your fellows.”
10And, “You, Lord, in the beginning, laid the foundation of the earth. The heavens are the works of your hands.
11They will perish, but you continue. They all will grow old like a garment does.
12You will roll them up like a mantle, and they will be changed; but you are the same. Your years won’t fail.”
13But which of the angels has he told at any time, “Sit at my right hand, until I make your enemies the footstool of your feet?”
14Aren’t they all serving spirits, sent out to do service for the sake of those who will inherit salvation?
Psalm 2:7
7I will tell of the decree: Yahweh said to me, “You are my son. Today I have become your father.
What each tradition says
Rabbinic / Midrashic
According to Kohler and Hirsch, the Targum paraphrases Psalm 2:7 to remove the anthropomorphism of the Hebrew. The paraphrase reads, "Thou art beloved unto me, like a son unto a father, pure as on the day when I created thee."
Kaufmann Kohler, Emil G. Hirsch, Son of God (Jewish Encyclopedia, 1901-1906) — Article 'Son of God' (Targum paraphrase of Ps. ii. 7)
Patristic
Chrysostom holds that “You are My Son, this day have I begotten you” expresses nothing other than from the time that God is. He says that as God is said to be from the present time, so “Today” here seems to him to be spoken with reference to the flesh.
John Chrysostom, Homilies on the Epistle to the Hebrews (NPNF I/14; newadvent edition) — Homily 2, on Hebrews i. 3-5
Catholic
Haydock says these words are commonly expounded as the Son's eternal generation in eternity, yet they may truly apply either to Christ made man by his incarnation or to Christ risen from the dead, as St. Paul uses them in Acts 13:33. For the same Christ is the Son of God both ways.
George Leo Haydock, George Haydock's Catholic Bible Commentary (1859) — On Hebrews 1:5
Orthodox
John of Damascus says the Only-begotten Son, Jesus Christ, is begotten of the Father before all ages. He is Light of Light, true God of true God, begotten not made, consubstantial with the Father, through whom all things are made. His birth is without time or beginning.
John of Damascus, An Exact Exposition of the Orthodox Faith (NPNF II/9) — Book I, ch. 8 ('Concerning the Holy Trinity')
Reformed / Protestant
John Calvin says the begetting in Hebrews 1:5 must be understood relatively, not as Augustine’s eternity or perpetuity. Though Christ is eternally the Son, born before time, that has no connection to this passage. Here the respect is to men, who acknowledged Christ as God’s Son after the Father manifested him.
John Calvin, John Calvin's Bible Commentary (Calvin Translation Society; 'Commentary on Hebrews') — Commentary on Hebrews 1:5
Evangelical
John Wesley takes “This day have I begotten thee” to mean “I have begotten thee from eternity.” For Wesley, that eternal begetting is, by its unalterable permanency of duration, one continued, unsuccessive day.
John Wesley, Explanatory Notes upon the New Testament — Note on Hebrews 1:5