Prolegomena I: one eternal act in God; James 1:17; Ambrose and Hilary
First, then, this must be known and held: if we consider God's nature and perfection in itself, he does not first see one thing and then another, nor first decree or will this and then that. By one most simple act from eternity he has seen all things at once and decreed all things at once in himself. The firmest ground is Divine immutability — James 1:17: with God there is no change, or shadow of turning. Those words shut every change — in being or in working — out of the Divine nature. If we imagine God acting by turns in those operations understood as immanent in him, so that he first understands this then that, first wills this then another, we place in God a move from potency into act — which contradicts his most absolute simplicity. So Ambrose is right (or whoever wrote book 2 of On the Calling of the Gentiles, chapter 10): In God there is no accident, motion, new will, or temporal counsel; his thought is not varied by the inequality of changeable things; he comprehends all times and temporal things together with an everlasting, steady gaze. Hilary agrees: he denies piecemeal thoughts of things in God, and holds that all things stand with God in the even infinity of eternity (On the Trinity 12, p. 193). Every Scholastic and theologian I have seen assents — except one Vorstius, whose ravings are not even worth refuting in a theological school.
Latin
Primo igitur sciendum & tenendum est, si Dei naturam & perfectionem in se consideremus, illum non prius unum videre, dein aliud; neque prius hoc decernere aut velle, deinde illud; sed unico & simplicissimo actu ab aeterno omnia simul vidisse, omnia simul in se decrevisse. Ratio firmissima petitur ab immutabilitate Divina, Jacob. 1. 17. Apud Deum non est transmutatio, aut vicissitudinis obumbratio. Quae verba omnem mutationem sive in essendo, sive in operando a Divina natura excludunt. Nam si fingamus Deum quasi per vices agere in illis operationibus quae in Deo immanentes intelliguntur, ita ut prius hoc intelligat, deinde illud, prius hoc velit, deinde aliud; ponimus in Deo mutationem a potentia in actum: quae ejus absolutissimae simplicitati adversatur. Recte igitur Ambrosius, sed quicunque alius Autor fuerit, lib. 2. De vocat. Gent. cap. 10. Non est in Deo accidens, motus, aut nova voluntas, aut temporale consilium, nec cogitatio ejus cum rerum mutabilium inaequalitate variatur, sed cuncta pariter tempora & temporalia sempiterno ac stabili comprehendit intuitu. Adstipulatur etiam Hilarius, qui negat in Deo particulatas rerum cogitationes, atque asserit omnia penes Deum aequabili aeternitatis infinitate consistere, de Trin. 12. pag. 193. Assentiuntur Scholastici & Theologi omnes quos ego viderim, extra unum Vorstium, cujus deliria indigna sunt quae vel refutentur in Schola Theologica.