Mercury

In some OT passages, the Angel of the Lord is depicted in ways identical to, and distinctive to, Yahweh. Recently, I was listening to a debate in which a Muslim apologist attempted to neutralize that comparison by resorting to the familiar strategy of saying that's because the angel of the Lord is a representative of Yahweh, an emissary of Yahweh, so that he's been authorized to speak for Yahweh as if he himself is Yahweh. There are, however, some fundamental problems with that comparison:

i) Both Judaism and Christianity arose in pervasively idolatrous, polytheistic cultures. Both Judaism and Christianity arose in explicit opposition to pagan polytheism and idolatry. They go out of their way to draw an intransgressible boundary between the Creator and the creature. 

ii) Compare a human king to the crown prince. The reason his subjects, as well as his enemies, treat the crown prince with the same deferential fear as the king is because the prince is the next king. He's in line to succeed the king. So you had better treat him as if he's the king, you better treat him with due deference, because it will be too late to honor him after he assumes the throne if you dishonor him while he is still crown prince. Indeed, how is the best time to ingratiate yourself with him.  

Put another way, there's no natural difference between a human monarch and his emissary. An heir apparent is the king in waiting. That's what makes him the highest-ranking emissary, if the king dispatches his son on a military or diplomatic mission. BTW, the Bible treats the Messiah as crown prince and heir apparent. The firstborn, &c. 

But that's precisely where the unitarian analogy breaks down if Jesus and/or the Angel of the Lord is just a creature. They cannot be agents of God in the sense that a prince is the king's representative. So the analogy is vitiated by a categorical disanalogy. 

iii) Indeed, in heathenism, there was no categorical difference between divinity and humanity. That ranged along a continuum. Gods and goddesses could sire offspring by human mates. Their offspring were demigods. 

It's precisely that blurring of the lines, so easy in heathenism, that's intolerable in Judeo-Christian theism. That's why both Testaments bend over backwards to prevent any possible confusion. 

Ironically, it's because the Bible is adamantly monotheistic that the unitarian appeal to agency runs counter to Biblical monotheism when unitarians claim divine ascriptions to Jesus and/or the Angel of the Lord simply reflect the conventions of agency. Consider Mercury. An Olympian emissary. Speaks with the authority of Zeus. But, of course, Mercury is a god in his own right.

The Bible cannot allow rational creatures to be depicted in terms interchangeable with Yahweh against a cultural backdrop in which the audience is conditioned to think of heavenly emissaries as members of the pantheon. In paganism, there's a divine hierarchy. Some gods rank higher, some gods rank lower. Upstart gods overthrow the old regime. The facile notion that the Bible can maintain its uncompromising hostility to paganism while describing Jesus or the Angel of the Lord in terms indistinguishable from Yahweh would utterly sabotage the Bible war against idolatry, polytheism, and demigods, if, in fact, Jesus and/or the Angel of the Lord are merely creatures. Even Israel, despite all the injunctions and sanctions against idolatry, was constantly teetering on the brink of reversion to ancient Near Eastern heathenism.