Brother DHK,
As a primitive Baptist, I do disagree with Calvinism on the issue of justification (on most other points I am in agreement), however to be fair, being a former Calvinist, most mainstream Calvinists do not believe/teach that God is the author of sin. See quotes below for support of my assertion-
“ This distortion of positive-positive predestination clearly makes God the author of sin who punishes a person for doing what God monergistically and irresistibly coerces man to do. Such a view is indeed a monstrous assault on the integrity of God. This is not the Reformed view of predestination, but a gross and inexcusable caricature of the doctrine. Such a view may be identified with what is often loosely described as hyper-Calvinism and involves a radical form of supralapsarianism. Such a view of predestination has been virtually universally and monolithically rejected by Reformed thinkers.” (RC Sproul, Double Predestination)
"...God is not the author of evil.”( John Calvin, Concerning the Eternal Predestination of God, p.169, emphasis mine)
"CHAPTER 3; OF GOD’S DECREE
Paragraph 1. God hath decreed in himself, from all eternity, by the most wise and holy counsel of His own will, freely and unchangeably, all things, whatsoever comes to pass;1 yet so as thereby is God neither the author of sin nor hath fellowship with any therein" (1689 Baptist Confession of Faith)
Everything that happens is by God's will, however there is a permissive will and a non-permissive will. God's permissive will simply allows something to happen (this would be the case with sin), his non-permissive will directly causes something to happen by working in the creature (e.g. regeneration, good works as the fruit of the Spirit, etc.).
God's divine purposes cause even the most evil acts of men such as the crucifixion of Christ to work out for greater good. The crucifixion of Christ is the most act of men the world has ever seen, but God made the crucifixion into his redemptive plan for sin for his people, and the greatest act of love of all time.