They take this from Samuel Rutherford: God's will is determined from eternity toward every being, so conditionals are true by his will
They admit that they took this doctrine from the Scot Samuel Rutherford, and in the same place they lay down, as a most certain foundation drawn from him, the following. God's will is not undetermined, doubtful or suspended toward any being, whether actual or potential, and whether it is to be actualized at some time or never brought into act.
Nor does his will contain potentiality, suspension and the lack of a second act, as a created will does. By the eminence of its act it is determined from eternity with respect to every being.
From this they infer that God necessarily wills or does not will every possible sequence and connection of beings. So in every conditional proposition about a contingent future, it is true by God's definite and certain will that if this or that condition were in place, this and nothing else would follow.
Latin
XXXIV. Porro hanc suam doctrinam fatentur illi se sumpsisse a Samuele Retorforti Scoto, ex quo, ut certissimum fundamentum, ibidem supponunt: Quod voluntas Dei ad nulla entia, vel quae actu sunt, vel quae sunt in potentia, seu aliquando actuanda, seu in actum nunquam deducenda, sit indeterminata, dubia, aut suspensa, seu habens in se potentialitatem, suspensionem & privationem actus secundi, qualis reperitur in voluntate creata, sed quod sit per eminentiam sui actus, respectu omnis entis ab aeterno determinata. Unde colligunt, Deum necessario velle, vel non velle omnes concatenationes & connexiones entium possibiles, adeoque in omnibus propositionibus conditionatis de futuro contingenti, verum esse ex definita & certa Dei voluntate, quod si talis vel talis conditio poneretur, sequeretur hoc & non aliud.