It has been said that the investigation of Christian evidences is on the whole unsatisfactory, because the point to which it is intended to lead the inquiry is known beforehand. This objection is very much in accordance with the habit of mind which loves a considerable degree of uncertainty, and which wishes to make the first elements of truth a mere field for speculation. But if this objection be good, will it not apply to other subjects also? For instance, in mathematical studies we know very well as soon as a theorem is enunciated what the point is which the teacher intends to prove. We are not instructed how to demonstrate that the three angles of a triangle are equal to two right angles, in order that this should afterwards be in our minds a debatable question, but we learn the demonstration that this may thenceforth be held as an established and unquestionable fact. Just so is it as to the evidence for the records of our religion. We do not prove the genuineness of the New Testament books on any grounds of mere opinion, so that what seems established today by be overturned tomorrow, but we demonstrate it by evidence, which loses no part of its value by lapse of time, any more than time can weaken the force of a mathematical demonstration.
– S.P. Tregelles (from A Lecture on the Historic Evidence of the Authorship and Transmission of the Books of the New Testament, 1851)
Monothelitism is a Christological “heresy” which states that Christ has two natures (divine and human) and one will; this is in opposition to dyotheletism which states that Christ has two natures (divine and human) and two wills (divine and human). Why the scare quotes around heresy? First, monothelitism was indeed condemned in 681 at the Third Council of Constantinople making it a formal heresy. However, from a Protestant position, these Councils, while helpful, are not authoritative. This means there is the distinct possibility for the council-members to make an error and unintentionally condemn something that’s not actually contrary to Scripture (alternatively, they could affirm something that is contrary to Scripture).
Is monothelitism an actual heresy running contrary to the fundamental teaching of Scripture? I think that it depends on the relation between will and nature. The motivation for the Third Council of Constantinople was to combat monophysitism: the teaching that Christ has one nature. This certainly is an actual heresy, running contrary to the Scriptural teaching of the hypostatic union. The truths that need to be held in balance are that (1) Christ is one person and (2) he has two natures. This is where presuppositions come into play.
There are two broad camps when it comes to the topic of will: combatibilists and libertarians. Compatibilists generally describe the will as a property of one’s nature. An individual wills to do something out of what her nature is. On the other hand, libertarians generally describe the will as a property of the person. An individual wills to do something out of who she is and not out of what she is . Libertarians would say where there is a will, there is a person (inb4 “where there’s a will there’s a way”).
Now, suppose that you’re a compatibilist and you hear that Christ has two natures; it follows naturally that Christ has two wills. If He only has one will, that would mean He only has one nature which is heretical. Suppose instead you’re a libertarian and you hear that Christ is one person; it follows quite naturally that Christ has one will. If He has two wills, that would make Him two persons which is super heretical.
In other words, monothelitism is a heresy within the compatabilist framework and dyotheletism is a heresy in the libertarian framework. It seems to me that in order to maintain monothelitism as a true heresy, one would have to demonstrate that libertarianism is incoherent in its rendering of wills as properties of persons instead of natures.
 This is not to say that libertarians deny one’s nature playing into one’s decisions. Soft libertarians posit that one’s nature provides the range or spectrum of choices available to an individual in certain circumstances but does not ultimately arbitrate which decision is made. This is in contradistinction to compatibilism which typically posits that the range of decisions consists of one member. This entire paragraph is a broad generalization and I recognize the shortcomings of its brevity.
The chief point to keep in mind about a hypothesis, or theory, is that it is to be cast aside for one that is better the moment the facts indicate the existence of a better. It is the traceless and eternal war of facts against superstition, predisposition, bias, and error which science undertakes to wage. This inductive method is pursued in physical science everywhere, and in the social sciences also, in economics, civics, sociology, and now at length in ethics and religion. Evolution and criticism both erect their intellectual structures with the stones blasted in the wary of the inductive method.
We have no option as defenders of Christianity. Of all people, we must not turn our faces away from an enterprise which first of all sets out in quest of facts. We hold that eternal facts, the most solid of all realities, are the contents of our Christian faith. We maintain that the only adequate hypothesis to account for a vast mass of facts is the Christian hypothesis, and that verification in all its legitimate forms in the personal and moral realm may be applied to the hypothesis successfully.
– Edgar Young Mullins (from Why Is Christianity True?, 1905)