Within the context concerning the nature of causality relative to God, occasionalism is the position that there are no such thing as true secondary causes. Instead, God is the only true cause of anything. The position does not merely state that God is the final cause of all things, but rather, that God is the only cause of all things. Within Calvinistic theology, this ought to be relatively uncontroversial, with some qualification. A Calvinist may acknowledge that God is the final cause of all things, yet he still usually maintains that God uses secondary causation to bring about His intended will. Secondary causes have metaphysical reality. Yet for occasionalists, we cannot even coherently speak of secondary causes being used of God to bring about His ordained will.
Secondary causes simply do not exist. For example, God is predestining my current activity of typing on the keyboard. Those who believe in secondary causes argue that God brings about the intended goal of my typing through certainly mediated means. These mediate means involve whatever electrical and physical processes go into responding to my typing by causing words to appear on a computer screen. Yet the occasionalist will deny that God uses such means. At least, he will deny that such means have a real, independent metaphysical reality. Proximal correlation of one event with another does not, for the occasionalist, constitute true causality.
These ordained regularities are just that: regularities. And each regularity is ordained as a kind of 'tendency' in the world whose purpose is to help humans navigate the world intelligibly. When my fingerse hit the keyboard, for example, the occasionalist would deny that my fingers are actually truly causing the individual keys to become depressed and to signal to the computer to cause words to appear on the screen. Instead, these are mere coincidental regularities whose purpose is to help me navigate the world intelligibly. Furthermore, it is God who immediately and directly causes each key to become depressed, and it is God who directly and immediately causes words to appear on the screen.
While counterintuitive, to say the least, it is not immediately obvious how this can be theologically problematic. However, serious issues start to arise when we begin to think about the question of sin "causing" humans to be held guilty. In a world without causation, it is difficult to imagine how my sin can "cause" me to incur legal guilt before God. Rather, God would have to temporally co-ordinate the declaration of my guilt with each of my sins. And yet, it is not actually my sin "causing" me to become guilty. It is simply God's decree that is causing me to become guilty, since God is the only true cause of anything. This is an example of what in philosophy is known as 'nominalism.' Nominalism is the philosophical position according to which universals do not exist.
It is possible, of course, for one to hold to a 'nominalist' view of certain classes of objects or entities. Extreme occasionalism, I argue, entails what I will refer to as nomological nominalism. That is, someone is held forensically guilty before God, not on the basis of a universal Law which reflects God's character, but merely on the ground of God's bare decree. This is not biblical. God's moral Law flows naturally from, and necessarily reflects, His character. Whenever a human violates His character, it objectively "causes" them to become guilty before God. Indeed, God's very decrees are circumscribed by His nature. It is impossible for God to lie(Heb. 6:17-18). It is not merely the case that God could not lie even if He wanted to, but rather, God's character is such that it is impossible for Him to even want to lie. It is on the basis of this innate characteristic of God that it is objectively wrong for humans to lie, and it is why we objectively incur guilt when we do lie.
Pushed to its logical conclusion, occasionalism could necessitate a universe in which God could condemn something by bare decree without their having sinned, or could refrain from actually condemning them when they do sin. Many early Calvinists actually did hold to comparable views, and contemporary Calvinists rightly reject such views as seriously misguided. John Calvin and the early John Owen (The Death of Death in the Death of Christ-era) both believed that God did not have to forgive sin by means of an atoning sacrifice. He simply chose to do this, but could have chose to do otherwise had He wanted to. The later John Owen, in his work A Dissertation on Divine Justice, rightly recants this view and defends its opposite.