Jaco van Zyl, a “Biblical Unitarian,” wrote the following, which captures why
appealing to Isa 45:5 and other like-texts in “Deutero-Isaiah” poses a problem
for Trinitarian claims:
Trinitarians like James White argue that Yahweh
(Adonai) speaks to someone else who is also Adonai. However they want to look
at it, this is troublesome even to Trinitarian theology: If Yahweh is 3-in-1
God, speaking to another Adonai adds between 1 and 3 to the existing 3, leaving
us with between 4 and 6 Persons in one God. If, however, you add the second
Adonai to the first, then Yahweh is 2 and not 3 Persons, isn’t He (or should I
say they)? (Jaco van Zyl, "Psalm 110:1 and the Status of the Second
Lord--Trinitarian Arguments Challenged," in An E-Journal from The Radical Reformation: A Testimony to Biblical Unitarianism, pp. 51-60, here, p. 60).