God created all things ex nihilo.
Therefore, the ultimate source and origin of one’s will is God.
God’s foreknowledge is exhaustive, not contingent (as it is within certain systems, such as Socinianism).
Related to the above, God’s foreknowledge is active, not passive—that is, God decrees all things, including sinful acts, and such events will infallibly take place, and not simply foreknows events passively (as one finds within simple foreknowledge and other theories of foreknowledge).[1]
Therefore, God is the author of sin.
Only by explicitly rejecting at least one of the above premises can a Calvinist consistently claim that God is not the author of sin. However, all the following premises are part-and-parcel of Reformed theology.
Notes for the above
[1] On the relationship between God creating ex nihilo and one’s will, consider the following from Blake T. Ostler, The Problems of Theism and the Love of God (Salt Lake City: Greg Kofford Books, 2006), 410:
If the causes of our acts originate from causes outside of our control, then we are not free and cannot be praised for blamed for what we do resulting from those causes. If the causes of our actions are outside our control, then our acts that result from such causes are not within our control either. Thus, a person must be an ultimate source of her acts to be free. By an ultimate source, I mean that some condition necessary for her actions originates within the agent herself. The source of action cannot be located in places and times prior to the agent’s freely willing her action. The source of the action is the agent’s own will that is not caused by events or acts outside of he agents but from the agent’s own acts of will. The doctrine of creation ex nihilo is contrary to such a view of agency on its face. Consider that: (1) If a person is created from nothing, then he is never the ultimate source or first cause of her choices. If we assume that (2) all persons are created from nothing, then it follows from (1) and (2) that (3) no person is the ultimate cause or source of anything. This argument does not require any particular concept that God acts in relation to humans or brings about their acts through cooperative grace. All it requires is the notion of creation ex nihilo. If the libertarian demand that we must be the ultimate source of our choices to be morally responsible for them it sound, then God cannot create morally responsible persons ex nihlo. In some sense, persons must be co-creators, first causes, unmoved movers of their own wills, and the source of their own choices
