Per the Nicene Creed:
Eternally begotten of the Father
Per the Holy Bible:
In the beginning the Word already existed. The Word was with God, and the Word was God. He existed in the beginning with God.
Jesus answered, “I tell you the truth, before Abraham was even born, I am!”
For through him God created everything in the heavenly realms and on earth. He made the things we can see and the things we can’t see — such as thrones, kingdoms, rulers, and authorities in the unseen world. Everything was created through him and for him.
Though he was God, he did not think of equality with God as something to cling to.
We'll deal with "begotten" in two weeks. The point today is "eternally" and the assertion that, like the Father, there was never a time when Jesus didn't exist.
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Truth is...infinity and timelessness are things that make my mind get all wobbly, but for Yahweh to always be the Father, there has to have always been the Son. As it's been stated before, if God could be totally explained, he wouldn't be all that worthy of worship now, would he?