Their main foundation (Twisse): nothing is future before God decrees it, since futurity from eternity needs an eternal cause
Twisse, Rutherford, Voetius, Ames and others deny that God knows, before his free decree, what would contingently happen under some condition. The main foundation they rely on is that, on their view, nothing is future until God has decreed that it shall be. It follows that before God's decree nothing can be conceived or known as future.
They prove that nothing is future, absolutely or conditionally, before God decrees it, on these grounds. No cause can be imagined or thought of, other than God's decree, by which a thing passes from being possible to being future from eternity. Nothing temporal can cause what is eternal.
So the futurity that exists from eternity cannot be traced to anything that happens in time. It must have some eternal cause, and that can only be God's free decree. Twisse repeats this a thousand times in his dissertation on middle knowledge.
Latin
XLII. Porro praecipuum fundamentum quo nituntur Tuissus, Retorfortis, Voetius, Amesius, & alii, dum negant in Deo dari cognitionem contingenter futurorum sub aliqua conditione, ante liberum ipsius decretum, illud est quod, ex eorum mente, Nihil futurum est priusquam decreverit Deus illud esse futurum. Unde sequitur, ante Dei decretum nihil ut futurum concipi atque cognosci posse. Nihil autem vel absolute vel conditionate futurum esse, priusquam decernat Deus illud esse futurum, inde probant, quod nulla causa fingi vel cogitari possit, per quam res ex possibili ab aeterno futura fiat, praeter Dei decretum. Cum enim nihil temporale causa possit esse ejus quod aeternum est, ad nihil eorum quae in tempore fiunt, futuritionis, quae est ab aeterno, causa referri debet, sed necesse est ut ista futuritio causam habeat aliquam aeternam, quae alia esse non potest quam liberum Dei decretum: ut millies repetit Tuissus in sua dissertatione de scientia Media.