Perhaps because he formulated a plan to help explain why it's okay for the state to be blended with the Christian religion. Using his ideas, Christians can support and be apart of a state that uses force and governs overs those who are not necessarily believers. Those who believe these actions are antithetical to Christian morals, namely Anabaptists, usually malign Augustine.

>>18754724
To be clear, Augustinianism, Thomism, Lutheransim and Calvinism are all ultimately distinct traditions that come to different conclusions. Augustine believed in the efficacy of prayers to saints and the power of relics, and he submitted to the Roman Catholic Church in absolutely everything (putting him at odds with the Lutherans and Calvinists). Thomists preach that all are given sufficient grace to be saved (putting them at odds with Augustine's strict view of predestination). Later thinkers would redefine Thomistic grace to differ between sufficient and efficacious grace, but these are not the distinctions that Thomas himself made (and Thomists, like Augustinians, believe in absolute submission to the Roman Catholic Church as a visible institution). Lutherans believe in the resistibility of grace, and Luther himself insisted that Christ is physically present in the Eucharist (putting them at odds with Calvinists).

The reason Augustine exerts such an immense influence over these groups is because he was one of the first truly perspicacious Christian teachers in the west. The east is very different (they arguably always had a larger corpus to draw upon); but insofar as the west is concerned, Augustine was the first big commentator.

It also helps to remember that Luther himself was an Augustinian friar (and was therefore strictly taught scripture according to Augustine's views).