Does “in the highest” modify God or Glory? How would the implications change if it was one or the other?
That’s a helpful question! Often discerning what a prepositional phrase is modifying will have an impact on your interpretation of a passage.
But I don’t think it matters too much in this case; if glory is “in the highest,” it is because God is “in the highest.” So it could mean “glory to the God who is in Heaven,” or “glory be in Heaven to God.”
Any thoughts on this, @Nate-Davidson?
From the UBS Handbooks for the New Testament:
hupsista ‘the highest’, cp. on 1.32; en hupsistois goes with theo as indicating the place where God is (cp. RSV, NEB, and others) or with the phrase as a whole, indicating the place where glory is given to God (cp. BFBS, Rieu); the latter seems preferable because of the place of en hupsistois before theō and of the parallelism with epi gēs ‘on earth’.
Hope this helps,