"My roomate, who isn't even a believer, says he thinks that's covered in John 1:1 or something like that. Anyone care to look it up to check?"
John 1:1, "In the beginning was the Word, and the Word was with God, and the Word was God."
We're discussing the Trinity here, or the concept of trinity in general. John 1:1 says that Jesus is God, just as Jesus said in John 10:30. Now God's incarnation had a beginning. It was when Jesus was born. That event is also the beginning of the Trinity.
Where did you learn that?
It says that the Word was both God and with God and goes on in v. 14 to speak of the pre-existent Word becoming flesh, i.e., incarnating into the world as a human being. Ergo, Yeshua pre-existed His incarnation.
Indeed, He pops up quite often in the Tanakh (the OT) if one knows where to look: As the Angel (Messenger) of YHVH.
If Yeshua pre-existed His incarnation (as both Isa. 11:1 and Mic. 5:2 attest), then too the Trinity pre-existed Mary. And if Yeshua is the Root of Jesse as well as the Branch of David, then He is Mary's Father as well as her Son (cf. Isa. 9:6). And just as David, who was Yeshua's (fore-)father still called Him Lord (Psa. 110:1, Mat. 22:44f) because Yeshua was his Creator, so does Mary call Him Lord and God.
The Trinity, comprised of Father, Son, and Spirit, of Will, Word, and Breath, pre-existed everything else, including Miryam bat Heli. Miryam had the great honor of bearing God-With-Us into the world, but He created her--she did not create Him.