Hebrews 5:5-6 (ESV)
So also Christ did not exalt himself to be made a high priest, but was appointed by him who said to him,
"You are my Son,
today I have begotten you";
 as he says also in another place,
"You are a priest forever,
after the order of Melchizedek."
So Christ was appointed to be high priest, by the Father because no one appoints themselves to this task. I don’t know why, but this phrase “ You are my son, Today I have begotten you” is toying with my head. It shows the difference between the Father and the Son, and being God and being human. Today I have begotten you.
I begat my son some 8 years ago. Jesus is the only begotten Son of God. We thing of begetting as something that would have happened in time. But from the perspective of eternity it seems much different. How can God say this to the Son? My son sure wasn’t responsive on the day that I begat him. But eternity and time are as different as God and man. The Arians used to chant that “there was a time when he was not.” They couldn’t conceive of eternity as timeless. I don’t know that I can either. I try,. I understand that it is timeless. But everything I experience and know has reference to time. So the begetting of the Son is a timeless event, an eternal event. It is constant, and never ceasing. The Son always is just as the Father always is.
Well understanding this verse sure throws a monkey wrench into the idea of “pre existence.” For those outside of Utah, Mormons believe in a Pre existence, which is to say The Father begat us in heaven, and we waited as spirit children until God could find a crack addict to task with the responsibility of giving us a body, and raising us. Just thought I would throw that in as an afterthought.