Unitarians like Dale Tuggy claim the Trinity contradicts Biblical monotheism. It allegedly violates the logical law of identity. However, there are several problems with his accusation:
i) A logical contradiction is a logically stringent category. Yet the Bible doesn’t define God’s unicity in logically stringent terms. Strictly speaking, the Bible doesn’t “define” God’s unicity at all.
Yet you’d need a stringent definition, which is derivable from Scripture, as a precondition to generate a logical contradiction–or apparent contradiction.
ii) The most detailed explication of divine unicity occurs in Isa 40-48. That, however, isn’t logically rigorous. Rather, that involves a contingent historical illustration. It compares the true God with the pagan pantheon of the ANE. So it’s a fairly generic, limited comparison.
The true God is distinguished by his premeditation, creative power, exhaustive foreknowledge, meticulous, inexorable providence, and miraculous deeds. The gods of the heathen are nonentities because they lack these characteristics.
True deity is (informally) defined by the possession of these indicia while spurious deity is (informally) defined by the absence of these indicia.
It would be contradictory for the true God to lack one or more of these indicia. Likewise, it would be contradictory for a false god to embody one or more of these indicia. That’s the framework.
That, however, is not logically equivalent to the claim that only one party can embody these attributes. In Isaian terms, it’s not contradictory to say that more than one party could embody these attributes. To say Ashur is a false god because he fails to embody the attributes of Yahweh does not entail the proposition that Messiah can’t possess the attributes of Yahweh. Indeed, if Messiah did embody these attributes, then by definition, he’d be truly divine.
Tuggy likes to vaguely assert a logical contradiction, but I have yet to see him actually derive a contradiction from his Isaian prooftexts.
iii) In addition, this is not a bare logical possibility. For there’s evidence that Isaiah did, in fact, exercise that option.
Take the accession oracle in Isa 9:1-7. In this oracle the “crown prince” not only has royal titles, but divine titles (9:6). Even a liberal commentator like Goldingay admits that all four titles are theophoric titles:
In Isaian usage, one of Yahweh’s defining attributes is the fact that he’s a God who plans the future, and successfully implements whatever he plans. No one tells him how to plan the future (e.g. 14:24-27; 40:13-14; 45:9-11; 46:10-11).
In context, that’s a standard title for the divine warrior (par. 10:21; 42:13; cf. Deut 10:17; Jer 32:18), who leads his army into battle. Who leads his people to victory.
In context, that’s a divine title for the God who adopted Israel and David (e.g. 63:16; 64:7; cf. Deut 32:6; 2 Sam 7:14; Ps 89:26,29).
Prince of Peace
This is another military title (cf. Dan 8:11,25), which parallels El Gibbor. Both titles evoke the divine warrior motif.
BTW, even a liberal commentator like Childs admits that this is an eschatological oracle rather than a historic coronation text.