Per the Nicene Creed:
...the only Son of God...
Per the Holy Bible:
The king proclaims the Lord’s decree: “The Lord said to me, ‘You are my son. Today I have become your Father.'"
And a voice from heaven said, “This is my dearly loved Son, who brings me great joy.”
For this is how God loved the world: He gave his one and only Son, so that everyone who believes in him will not perish but have eternal life.
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For Yahweh to be eternally identified as "Father," there has to have eternally been a "Son". That loving, authoritative/submissive relationship has always been present in the Godhead and is beyond our complete comprehension. How can Jesus be both God and Son? That is a mystery clearly taught in Scripture, but never fully explained.
Truth is...if I could completely understand God, I don't think he'd be worth worshiping.