Full Idea
Suppose that God wills that John kiss Mary now. God's willing thereby necessitates the truth of 'John is kissing Mary'. But God's act is not a truth-maker for this judgement.
Gist of Idea
God might necessitate that something happen, but He is not the truth-maker for it
Source
Barry Smith (Truth-maker Realism [1999], p.6), quoted by Fraser MacBride - Truthmakers 1.2
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.5
A Reaction
The point is that truth-making relates to the fact that it happened, not what necessitated it to happen. But Armstrong might reply that his truth-maker 'necessitation' primitive is not the kind of necessitation found in worldly relations.
Related Idea
Idea 18468 Armstrong says truthmakers necessitate their truth, where 'necessitate' is a primitive relation [Armstrong, by MacBride]