Is Perseverance Synergistic?

No—and the whole worry rests on treating “mediated” as if it meant “synergistic.” They are not the same thing. Synergism claims the human will contributes something to salvation, cooperating with grace as a second agent; that the Confessions reject flatly (FC SD II). Mediation asks a different question: not who acts, but by what instrument God acts. And God has bound His preserving grace to Word and Sacrament (AC V), the external means through which the Spirit is given—never apart from them (SA III.VIII).

In those means, God is the actor. Faith does not cooperate there; it receives—the empty hand, the mouth that eats bread it did not bake. Receiving a gift adds nothing to the gift. So preservation runs through the means without running through you as an agent: it is grace the whole way down, and He “is able to protect you from stumbling” (Jude 24, NET). Neglecting the means, then, is not the failure to perform your half of a joint project. It is shoving away the very Spirit who was holding you.