Randolph Sinks Foster, in his book, Objections to Calvinism (1852) writes:
[The Confession of Faith states,] “God, from all eternity, did, by the most wise and holy counsel of his own will, freely and unchangeably ordain whatsoever comes to pass; [and now your disclaimer,] yet so as thereby neither is God the author of sin, nor is violence offered to the will of the creature.”
But this disclaimer [God is not the author of sin] by no means relieves my embarrassment — it greatly increases it, by placing you [Calvinist brother] in the attitude, to my mind, of believing a palpable contradiction, namely, that God did cause all things, sin included, yet in such a way that he did not cause sin.”
[In other words, it appears that 2 + 2 = 4; but if you think that 2 + 2 = 4 then you are mistaken. 2 + 2 does not equal four. God ordained whatsoever comes to pass = 2, sin has come to pass = 2. Therefore, it appears that 2 + 2 = God is the author/cause of sin; but let me assure you that you are mistaken and wave my hand in dismissal. God is not the author/cause of sin.]
Foster continues, “It is as though you should say, Lycurgus made all the laws of Sparta, yet in such a way, that there were many laws of Sparta which Lycurgus did not make. But supposing that the absurdity does not strike your mind [Calvinist brother] with the same force it does mine — or of course you could not embrace it — I shall more particularly present the reasons; and perhaps you can assist me in my conclusions.
I reason thus, and the process is exceedingly brief and simple: “God decreed whatsoever comes to pass;” but sin comes to pass; therefore, God decreed sin. “What God decrees, must necessarily come to pass;” but he decreed sin; therefore, sin necessarily comes to pass.
“God’s decree is the necessity of things;” but sin is something; therefore, God’s decree is the necessity, or necessitating cause of sin. God’s decree, being from eternity, precedes all things; and whatever is in time results from God’s decree, as its cause; but sin is in time; therefore, sin results from God’s decree, as its cause [author].
Let me particularize now. The [Calvinist] doctrine is, that God decreed, from eternity, whatsoever comes to pass in time — and that according to his own good pleasure — every particular thing, event, and act. I must insist, according to this [Calvinist doctrine], that he decreed the sin of every sinful man — nay, each particular sin of each particular man, and all the sins of all men, long before the human race was created.”
Hence, the Westminster Confession contains a palpable contradiction namely, that God did cause all things, sin included, yet in such a way that He did not cause sin. To reconcile the obvious, the Calvinist simply waves his hand and says God is not the author of sin.