Vincent Cheung is a heretic, because he holds that God is the author of sin. But first, what is the meaning of the phrase "author of sin," and what does Cheung mean when he say that God is the "author of sin"?
The traditional meaning of the term "author of sin" is understood in relation to its use in the Westminster Confession of Faith 3.1, which 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; yet so, as thereby neither is God the author of sin, nor is violence offered to the will of the creatures; nor is the liberty or contingency of second causes taken away, but rather established. (WCF 3.1)
Therefore, the phrase has historically meant that if God is the "author of sin," then God makes a person sins directly, either through violating the will of the creature, or by taking away the liberty and contingency of second causes.
In understanding the historical definition of the phrase "author of sin.," we can see that Cheung redefines the phrase, while he actually holds to its historic meaning. Cheung therefore does not formally hold to God being the "author of sin," while he holds to God being the "author of sin" materially, since he affirms the essence of the traditional meaning of the phrase "author of sin," as I have stated here.
Why is Cheung holding materially to the phrase "author of sin" wrong? It is wrong because it makes God into a schizopreniac, a monster, or an untrustworthy being. I had mentioned this in some way in a previous post: Vincent Cheung and the Author of Sin, but I would like to explicate it further to nail down the argument and make it as clear as possible.
Here is the argument for why God being the "author of sin" is wrong (P = Premise, C = Conclusion):
P1: If God is the author of sin, he directly causes people to sin.
P2: God is the Author of sin (Cheung's assertion)
(From P1 and P2) C1: God does actions that directly makes people sin.
P3: Directly making people sin is an evil action.
(From C1 and P3) C2: God does evil actions.
P4: God is by nature good.
(From C2 and P4) C3: God who is by nature good does evil actions.
(from C3) C4: God by nature does something contrary to His nature.
(From C4) Possible conclusion 5a: That is impossible, thus one of the previous premises must be wrong. P1 and P3 are true by definition, so P4 must be false, and God actually is evil and thus a monster.
(From C4) Possible conclusion 5b: God can will to do something contrary to his nature.
(from C5b) C6: God's actions and God's nature are not necessarily linked.
(from C6) C7: If God's actions and nature are not necessarily linked, then God can promise one thing and do another. God's faithfulness is undermined.
(from C7) C8: God is therefore untrustworthy, a schizopreniac, or both.
P5: The biblical God is faithful and trustworthy and good and not schizopreniac
(from P1, P5, C5a and C8) C9: The God who is the "author of sin" is not the biblical God, and this teaching that he is the "author of sin" attacks the character of God.
This argument about the gross error, even heresy, of holding that God is the "author of sin" deals with ontology, not ethics or epistemology. Thus, it deals with the being, the attributes of God. That is why the irrational nominalism of Cheungians is a next to useless counter argument. If the logical implications for God being the "author of sin" is an assault on the character of God, then to repeat over and over again that "God is by definition good" (which is an ethical argument), is useless.
Finally, Cheung's assault against compatibilistic free will and soft determinism, which props up his argument that God is the "author of sin," is baseless, as I have shown in my article "God, the Author of Sin and Metaphysical Distanciation: A Rebuttal to Vincent Cheung's Theodicy." Cheung's position is spiritual poison, and it has many practical implications for life, the main ones which I have pointed out in another article here, entitled "Some Practical Problems with Cheung's heresies."
Lastly,Cheung's determinism is not the same as Gordon Clark's determinism. Gordon Clark held that God is not the "author of sin" and that secondary causes are real causes so that the contingency and liberty of second causes are established, as the WCF 3.1 states. Clark was a presbyterian, and he would definitely reject Cheung's heretical teaching of God being "the author of sin."