No true Scotsman is a form of (informal) logical fallacy, of the type where having set up definition X of something, e.g. “A scotsman is blah, blah, blah”, and faced with a particular example, “Well then, so-and-so is a scotsman, based on your definition”, the interlocutor moves the goalposts, “Well, no true scotsman would (insert characteristic of aforementions so-and-so”), thus excluding them from the refined definition X1.
A similar thing is going on in Hilary’s debate(s) with (unnamed) opponents, which he tackles in Book 5 of his De Trinitate. Having spent Book 4 tackling the confession drawn from the Letter of Arius, and arguing from the Old Testament that the Son is God, he then spends Book 5 arguing that the Son is verus Deus, “true God”, against the contention that the Father alone is verus Deus.
In sections 25-31, Hilary turns his attention to the combination of Deuteronomy 6:4, “Hear O Israel, the Lord your God is One” (audi, Israel, Dominus Deus tuus unus est) Isaiah 65:16 “they will bless [you] the true God” (benedicent [te] Deum verum). Modern translations render the Hebrew of that verse differently (So that he who blesses himself in the land shall bless himself by the God of truth (ESV) Whoever pronounces a blessing in the earth will do so in the name of the faithful God (NET)), but let’s stick with the Latin for now.
Hilary’s response is very interesting. Firstly, he suggests that te is an addition, and a very problematic one. “if te is read, the pronoun appears to signify a second person; otherwise if the pronominal word is absent, then the noun[1] refers to the speaker of the statement itself.”[2] Now, actually, Hilary is very interested in two related questions every time he does this kind of exegesis: who is the Speaker, and who is the Addressee. Furthermore, when the Speaker is God in the Old Testament, and the Addressee is called ‘God’, Hilary understands this to be indicative of the difference in the Trinitarian persons, for otherwise God should (properly speaking) refer to himself in the First person alone.
The next thing Hilary does is to quote Isaiah 65:13-16 at length. He does this because he considers proper interpretation to depend upon proper contextualisation. (To those who think the Ancients didn’t know anything about exegetical method, take note!) The introduction of this passage clearly indicates that the Lord is speaking through the Prophet. Hilary further argues for why the adjective ‘true’ is supplied here, and argues that it is in reference to the ignorance of the Jews who worshipped God simpliciter, not as Father, and so were ignorant of the Son and did not recognise him as God in his incarnation.
Then (wait for it…) he commences a clause-by-clause analysis of the whole passage, giving his thoughts on what each element means. Especially important is his understanding of verse 15, “You will leave your name for a rejoicing unto my elect, but the Lord will kill you” [3]. Anyway, differences aside, Hilary interprets the first part of this in terms of Romans 2:29 and the elect, i.e. Christian believers, as the new Israel. The second part, “The Lord will kill you”, he interprets in line with his principle that a mention of God by God must indicate a difference of persons. Thus the Dominus who will kill is the Son. This allows him to take the ‘new name’ of Isaiah 65:15b, “but my servants will be called by a new name”, to refer also to Christ. All of which leads to the key verse, 65:16. Having established by the context that the God referred to within the passage is God the Son, the words verum Deum refer not back to God the Father, Deus solus verusque, but to God the Son. Thus one cannot use the adjective verus to exclude the Son, for the very verse they call upon to do so, actually refers to the Son as Deus verus!
[1] i.e. Deum
[2] Personae enim alterius videtur esse pronomen, ubi te est: caeterum ubi pronominis syllaba non erit, ibi ad auctorem dicti refertur et nomen. V.26 with a slightly freer translation.
[3] Relinquetis enim vos nomen vestrum in laetitia electis meis, vos autem interficiet Dominus; again, significantly different from Modern translations, such as “You shall leave your name to my chosen for a curse, and the Lord God will put you to death (ESV)”; “ Your names will live on in the curse formulas of my chosen ones. (NET)”