In your Systematic Theology, you say that sanctification is synergistic, because we cooperate with God, even though he is the cause of our deeds, the cause of our good works. But, in this sense, couldn’t we say that the conversion is synergistic as well? I’m not talking about regeneration or justification, but about the act of believing. Because, even though God is the author and the cause of our faith, we exercise the faith, like in sanctification. Wouldn’t it be better to say that even our sanctification is monergistic?
Let us use chess as an analogy. All analogies have their limitations, but as long as we focus on what an analogy is meant to illustrate, it can be useful.
There are two “levels” of reality in a chess game:  The actions of the players, and  The relationships between the chess pieces. If we are talking about #2, then “knight takes pawn” makes sense. But when we say this, of course we do not mean that the knight moves itself and removes the pawn from the chessboard. We assume that the knight has no inherent power to move itself, but we assume that a player caused the movement. However, as long as we are speaking on level #2, we do not need to mention the player, even though he is a necessary part of the whole picture. But when the topic switches over to a discussion of cause, to level #1, then “knight takes pawn” makes no sense as a description of cause, since it is not a description of cause. We must talk about the player.
Now, to bridge this analogy over to our topic, let us add a few things to the chess game: [A] Suppose the chess pieces are in fact sentient — intelligent beings with awareness, thought processes, and feelings. [B] Suppose any event on the chessboard, any action associated with the chess pieces, must be entirely caused by a player. This includes the thoughts and feelings in the chess pieces. [C] Suppose that some actions taken by the player on the chess pieces involve an awareness in the chess pieces, and some actions do not.
Given B, it is conceivable that we might have no interest in distinguishing the two kinds of actions in C. But suppose we are interested in making a distinction, then we can call the first kind C1, and the second kind C2.
We are now ready to apply this to the topic of divine sovereignty and salvation.
If the topic is metaphysics, or causation, then terms like “synergism” and “secondary cause” make no sense. They are meaningless and useless. When the main topic is divine sovereignty, we are indeed talking about metaphysics, about causation. This means that I oppose the traditional doctrine on the matter. It is absurd to say that God’s sovereignty does not take away but rather establishes “the liberty or contingency of secondary causes.” Of course liberty and contingency are taken away. They, along with the very idea of secondary causes, are entirely destroyed and rendered meaningless.
If the topic is the relationships between created objects, then the term “secondary cause” still makes no sense, since if the discussion is limited on this level, then the “cause” is in fact not “secondary,” and what is “secondary” is not really the “cause.” I sometimes, but rarely, use the term to accommodate custom (so that people know what I am talking about, although, since the term itself is nonsense, I wonder if anyone really knows what the doctrine is talking about!), which I try not to completely overturn, although one can hardly blame me if I were to do so. In any case, I usually make some qualifications to avoid too much confusion.
My use of the term “synergism” is also to accommodate custom.* There should be some way to distinguish between C1 and C2. All things are caused by God, but things like election and justification are not associated with any awareness or feeling in man, whereas sanctification — such as resisting temptation, or even moving your lips to pray — involves some conscious effort, some awareness and feeling in man. (Note that in my Systematic Theology, I also use consciousness of effort as the reason to make a distinction for sanctification.) In C2, even the effort and the awareness are directly caused by God, so that metaphysically speaking, man does not in fact contribute or cooperate — creatures never contribute or cooperate in the metaphysical sense, but only in a relational sense.
Thus some distinction between justification and sanctification is warranted. Nevertheless, your question raises the issue as to whether synergism is the best term to describe this distinction, since the idea of “energy” is involved, and man does not in fact contribute any inherent energy from within himself to cooperate with God. Even any “energy” that is placed in man must be moved by God to function. In sanctification, God causes man to cooperate with the divine precepts while causing an awareness of effort in man.
As for whether there should be a distinction between conversion and sanctification, I see your point, but a distinction could be warranted. This is because, at sanctification (e.g. when one resists temptation), a person already has faith, but when a person is converted, he receives faith — there is no spiritually good effort to receive faith. In terms of causation, it could be argued that there is no difference — all things are caused directly and solely by God. But in terms of relation between created things, there is a difference. In sanctification, God has already installed a godly disposition in man, and causes him to be aware of his efforts.
The bottom line is that there is no real synergism in any action taken by creatures, but if we need a word to signal a distinction between two kinds of events caused by God — one without man’s awareness, the other with man’s awareness — then “synergism” is one option, although it is arguably an imperfect one.
* To illustrate, although we speak of the “inspiration” of Scripture, 2 Timothy 3:16 refers to the “expiration” of Scripture, that God breathed out the words of the Bible. However, the word “inspiration” has been so used to denote the revelation and inscription of the words of Scripture that it is not confused with the idea of breathing in something. Nevertheless, in a theological context, both inspiration and expiration ought to be defined and consistently used, as I have defined monergism and synergism, and have used these terms in a consistent manner.