The Significance of this question
This is an incredibly important and significant question that must be answered biblically. This is not just for academic scholarship. I think all pastors need to know this, and all Christians should too. Many evangelical Christians often just gloss over this whole ‘begotten’ thing of Jesus. As long as we know that Jesus is the Son of God, it doesn’t matter anymore. That applies for pastors too. I beg to differ here. It’s not enough just to know that Jesus is the Son of God, but the real question is Jesus equal with God, and in fact God? If Jesus is not eternally begotten, and only at some point he is begotten into being, then he is a creature, a creation of God, like man, as if a super good clean and righteous man. That’s what liberal theologians believe and want you to believe. In that case, the entire Christianity will collapse. The redemption of sins of mankind has just been trashed, because no creature can redeem the sins of fellow creatures. We must have a God-Man to save us, to redeem us.
Augustine’s Trinity Concept
I was so intrigued with this question that in my Christology Class, I determined to write a paper entitled “Is Jesus the Son of God equal with God the Father?” I am pretty satisfied with that paper, especially I quoted from Augustine who wrote a groundbreaking statement:
Augustine expounds on the eternal generation based on John 5:26 “For as the Father has life in himself, so he has granted the Son also to have life in himself.” That is not human life, but rather God’s life, immutable and dependent on nothing outside God (Tract 19.8, 149).”
The Father has life in himself. No creature has that, all life in humans is given by God, at the time of creation for Adam & Eve, and for us, at the time of conception in our mothers’ wombs. That’s an incredible concept. Life is in himself! And Jesus is granted to have life in himself too! That’s what it means ‘begotten’. In English language, begotten is an old language, like father begets son, means biologically, father and mother’s sexual union produces a pregnancy and eventual a baby, and that’s begetting. But for God in the Trinitarian, begetting means God grants that life in himself. You see mother and father begets a child, but ultimately that life comes from God of creation. God created that life in the mother’s womb. But for Trinity, the mystery is the word ‘grant’ –bottom line, Jesus life was not ‘given’ by the Father, and definitely not created by the Father, but God merely grants that Jesus has life in himself.
The Son and the Father do not only possess life, but life in himself that was given by none. That granting of life in himself to the Son is what Augustine calls ‘begetting’ or ‘eternal generation’. (Tract 19.13, 152). Augustine calls it “has been given (grant) ” is roughly equivalent to “begotten”. 
I love the way Augustine made it communicable the meaning of ‘begotten’—that’s what it means Jesus was begotten.
When was Jesus begotten?
I enjoyed Dr. Charles Lee Irons’ article https://credomag.com/article/the-only-begotten-god/ in Credo Magazine recently.
I am particularly blessed when he dealt with the “Today I have begotten you” issue with Jesus as the Son to God the Father, and yet we know from Nicaean Jesus is eternally begotten. There seems to be a contradiction. However, he quoted Romans 1:3-4 that saved the day for me.
Romans 1:3-4 (ESV) 3 concerning his Son, who was descended from David according to the flesh 4 and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord.
Jesus was “declared” to be the Son of God by the Spirit by his resurrection from the dead. He was always the Son of God, but officially, it was declared at his resurrection. So we can apply that to “begotten”. ‘Today I have begotten’ refers to the day of resurrection, like it was announced, declared, but he was always begotten.
This is only a beginning of the journey to understand more about the Trinitarian God. Jesus is God equal with the Father.
 Keith E Johnson, What Would Augustine Say to Evangelicals Who Reject the Eternal Generation of the Son? SBJT 16.2 (2012): 26
 Ibid., 29
 Charles Lee Irons- The Only Begotten GodEternal Generation in the Nicene Creed, Credo Magazine