Actually, Badfish has a point that answers your question. Let's just elaborate on what he mentions. From around the mid-second century, a theological viewpoint was emerging that attempted to struggle with the obvious implications of Christ's human as well as divine status. This viepoint suggested that, despite being born human, Jesus Christ somehow was elevated to some sort of divine or semi-divine status at His baptism. Something happened when the Holy Spirit invaded His Person which transformed Him into a bonafide Son of God. He was not born the Christ or Son of God [despite what Lu 2:11 said], but became such at that time.
This viewpoint came to be called Adoptionism, and over the next two centuries would evolve into several sub categories. One category, for instance insisted that the "Christ" was a divine creation of God, the "first of God's creations" who came into the human Christ at the baptism, and who left Him and went back to heaven at the crucifixtion, leaving the human Jesus to die physically.
The main difference between this viewpoint and Orthodoxy, was that whereas the Adoptionists saw Jesus Christ as an elevated human being who became the Son of God, the Orthodox position was that Jesus Christ, who always was the Son, was actually demoted, or devalued as a Person when He emptied Himself and and became human.
One such adoptionist, who created much controversy in the Church by the fourth century, was Arius. His viewpoint approximates the current Watchtower theological structure, which sees the human born Jesus changed in status from mere human to Son of God at the baptism. I am not sure what scripture Arius used to sustain his own theology, since much of his own writings have perished, but the one made by the Watchtower, Gal 4:26, is hardly contextual with the Person of Christ.
Naturally, being hide bound "organizationalists" the Watchtower sees the NT only through its own constructed "organizational" lenses. The "Jerusalem above"is precisely defined as "OUR mother"and not that of Jesus. [Know anyone who created his owm mum??] What then is this "Jerusalem above?
Dunno.
Paul himself does not explain at length what it is, thus there are several evangelical explanations. One, I believe, is that it is the Christian Hope. Paul tells us that whatever it is, it is an allegory [Greek "allegoria"]. He tells us of two women who represent two mountains. Hagar represents Mount Sinai from whence came the Mosaic Law which burdened the physical Jews with the hope of salvation through endless OBEDIENCE to its edicts.
But, Sarah we are told, represents the Jerusalem above, from whence comes the Christian hope of salvation through FAITH in Christ as a Person, in both His human and Divine status.