In his study of Marius Victorinus (b. ca. AD 280; d. ca. AD 370), David Bradshaw stresses that some “Neoplatonic reflections about being failed to reach the philosophers of the Middle Ages” and that “the concept of esse as act of being had to be reconstructed—in a very different way—by Aquinas.”
Bradshaw argues that Victorinus’s Trinitarian theology contributed to the developing of the idea of existence as a kind of activity and that “the most important channel for Victorinus’s influence was Boethius (480-525).”
According to Bradshaw, the distinction between esse and id quod est presented by Boethius in De Hebdomadibus has its source in Victorinus.
However, Bradshaw points out that important differences exist between Boethius and Victorinus. On the one hand, Victorinus is doing theology and trying to adapt previous Neoplatonic reflections about being to the doctrine of the Trinity. Boethius, on the other hand, is writing a tract on general ontology, thus incorporating Victorinus’s speculations into the metaphysical analysis of sensible substance. Boethius thus conceives the relationship between id quod est and esse on the static model of participation rather than the more dynamic model of Victorinus in which esse is conceived as a kind of activity.
Bradshaw concludes by saying that “It is remarkable that Aquinas, without any knowledge of these Neoplatonic antecedents, was able to find in the De Hebdomadibus an inspiration for his own conception of act of being.” 
↑ David Bradshaw, “Neoplatonic Origins of the Act of Being,” The Review of Metaphysics 53 (1999): 383-401.