One word that occurs in ecclesiastical circles is the word "synergy". It literally means a working together and refers to the work of Salvation. Synergy is our co-operation with God in the business of Humanity's salvation in the Church. Many Protestants are monergists in one way or another, meaning that they believe that God does all the work and that there is nothing we can do to bring about our salvation. There are some Protestants who would call synergists Semi-Pelagians, but that is not necessarily true. The Semi-Pelagians believed that the faith that we need to be saved begins in us. This is a position condemned at the Council of Orange in AD529. Orthodoxy tells us that it is God who begins the work of salvation of each of us by giving us grace to repent, i.e. to turn to Him.
Synergism, correctly stated, says that God first takes the initiative in our salvation but we then have a part to play in cooperating with that grace. Our salvation is accomplished by the free-will of God and Man.
When we start talking of two wills, one human and one divine, then we are beginning to talk about the question of Monothelitism which was condemned as a heresy by the Sixth Oecumenical Council. Monothelitism, Monoergism (not the same as Monergism which is the opposite of Synergism) and their parent Monophysitism are all heresies about the person of Jesus Christ. Orthodox teaching arising from the Oecumenical councils tells us categorically that
Our Lord Jesus Christ the Son of God, is God and Man; God, of the Substance of the Father, begotten before the worlds: and Man, of the Substance of his Mother, born in the world; Perfect God, and Perfect Man: of a reasonable soul and human flesh subsisting; Equal to the Father, as touching his Godhead: and inferior to the Father, as touching his Manhood. Who although he be God and Man: yet he is not two, but one Christ; One, not by conversion of the Godhead into flesh: but by taking of the Manhood into God; One altogether, not by confusion of Substance: but by unity of Person. For as the reasonable soul and flesh is one man: so God and Man is one Christ. (from the Quicunque Vult)The teaching which arises against the three "mono"s is that Our Lord possesses two natures, two wills and two "energies" - an "energy" being a capacity to do work - in each case, one is human and one Divine.
It is the orthodox condemnation of Monothelitism (that Our Lord, despite His two natures only had a divine will) in the Sixth Oecumenical Council that is relevant to synergism. In order for the Lord to work out our salvation, it required Him to be in possession of both a human and divine will, not just the divine will. That is significant. In order to save us, Our Lord had to be fully human and this includes the human will. He is tempted as we are, yet without sin, and the human will He possesses is
clearly not affected by the sin endemic in humanity, namely Original Sin.
We still have to ask ourselves, why two wills? why not be content with just the Divine if that is enough to save a human being? In our salvation, our wills are going to be reunited with God, this is clear. Our wills are to be perfected as part of our transformation in God. The perfection of our wills comes through repentance, i.e. an alignment with the Divine will. If God makes us want to repent, then that is a demonstration that there is no need for the human will - there exists only the Divine Will. Yet this is not the information that the rejection of Monothelitism gives us - the Human Will cannot be freely made to want - there is not a single will, but two. The Divine Will is that of Love which does not insist on its own way, and suggests the freedom of will that human beings possess to choose God and repent.
For me, the Sixth Oecumenical Council suggests that synergism is the truth of our salvation. I doubt that this will convince the die-hard Monergist. Yet, this is a question that anyone who subscribes to the Sixth Council must answer if they wish to continue their Monergism. If the human will is not free to choose God, then why does Our Lord possess a human will?