Excerpts from R. C. Sproul – What is Reformed Theology?:

The doctrine of justification by faith alone was debated during the Reformation on the deeper level of monergistic regeneration. This technical term must be explained. Monergism is derived from a combination of a prefix and a root. The prefix mono is used frequently in English to indicate that which is single or alone. The root comes from the verb “to work.” The erg of monergy comes into our language to indicate a unit of work or energy. When we put the prefix and root together, we get monergy or monergism. Monergism is something that operates by itself or works alone as the sole active party. Monergism is the opposite of synergism. Synergism shares a common root with monergism, but it has a different prefix. The prefix syn comes from a Greek word meaning “with.” Synergism is a cooperative venture, a working together of two or more parties.

When the term monergism is linked with the word regeneration, the phrase describes an action by which God the Holy Spirit works on a human being without this person’s assistance or cooperation. This grace of regeneration may be called operative grace. Cooperative grace, on the other hand, is grace that God offers to sinners and that they may accept or reject, depending on the sinner’s disposition.

Monergistic regeneration is exclusively a divine act. Man does not have the creative power God has. To quicken a person who is spiritually dead is something only God can do. A corpse cannot revive itself. It cannot even assist in the effort. It can only respond after receiving new life. Not only can it respond then, it most certainly will respond. In regeneration the soul of man is utterly passive until it has been made alive. It offers no help in reviving itself, though once revived it is empowered to act and respond.

Here we reach the ultimate point of separation between semi-Pelagianism and Augustinianism, between Arminianism and Calvinism, between Rome and the Reformation. Here we discover whether we are utterly dependent on grace for our salvation or if, while still in the flesh, still in bondage to sin, and still dead in sin, we can cooperate with grace in such a way that affects our eternal destiny.

Arminianism reverses the order of salvation. It has faith preceding regeneration. The sinner, who is dead in sin and in bondage to sin, must somehow shed his chains, revive his spiritual vitality, and exercise faith so that he or she may be born again. In a very real sense regeneration is not so much a gift in this schema as it is a reward for responding to the offer of grace. The Arminian argues that in this universal prevenient grace is primary, in that God first offers grace for regeneration. God takes the initiative. He makes the first move and takes the first step. But this step is not decisive. This step may be thwarted by the sinner. If the sinner refuses to cooperate with or assent to this proffered grace, then grace is to no avail.

In a related article Alan Kurschner writes:
The “Calvinist vs. Arminian” debate is substantially a debate between what is called “monergism” and “synergism.” There is no third option (unless one is willing to affirm Pelagianism). For those who are new to the Calvinist-Arminian debate, the following is a primer on the two perennial branches of theological systems in Christianity. Or to put it another way, there are two very different ways for believers to view their salvation.

In general, the first type (the Arminian-Synergist) affirms what is called “synergism.” Synergists believe that two forces in the universe are necessary to bring about regeneration in the life of the sinner. In specifics, the two forces at work (cooperation) that are necessary to bring about regeneration, or spiritual life, is the will of man and the Holy Spirit (grace).

To put it another way, the work of the Holy Spirit is dependent on the creature’s will, hence, “synergism” (working together). These individuals will sincerely say, “I believe in grace alone.” But in reality, they believe that grace is not alone (sufficient), but that man’s will is necessary for regeneration to be effective.

It could be said that these individuals are “functional” Arminians because even though some will deny the label, their theology functions synergistically (thus, how they identify themselves is inconsistent with what they teach and believe).

The second group of believers (the Calvinist-Monergist) affirm what is called “monergism.” Monergists believe that there is only one force in the universe (grace alone) that brings about regeneration in the life of the sinner. In specifics, because of the deadness of man’s spiritual state, his moral inability, the Holy Spirit performs the miracle of spiritual resurrection (regeneration) in that person, hence, “monergism” (one work). Grace is sufficient to be effective, and does not depend on some action of man.

In other words, the Holy Spirit does not merely whisper in the hardened sinner’s ear, hoping that the rebel sinner will “cooperate”; rather, while the sinner is in a state of hardness and rebellion, the Holy Spirit penetrates in the will of man and performs the miracle of spiritual life (regeneration). That is grace alone. Faith does not precede regeneration, regeneration precedes faith.

But because of his great love for us, God, who is rich in mercy, made us alive with Christ even when we were dead in transgressions– it is by grace you have been saved. Ephesians 2:4-5

Yet to all who received him, to those who believed in his name, he gave the right to become children of God— children born not of natural descent, nor of human decision or a husband’s will, but born of God.” John 1:12-13

He who belongs to God hears what God says. The reason you do not hear is that you do not belong to God.” John 8:47

Arminians cannot affirm monergism (grace alone); they must always have the creature’s will as the final determiner of their destiny, not God. Inconsistently, Arminians pray (without knowingly) Calvinisticly, “God, change my unbelieving relative’s heart.” I have never heard them pray, “God, only whisper in my relative’s ear, but don’t change their heart unless you’ve been given permission.” But the Calvinist prays and affirms biblical truth consistently.

John Hendryx:
Synergists teach ‘… and as many as believed were ordained to eternal life.’ but the Bible teaches ‘And as many as were ordained to eternal life, believed.’ (Acts 13:48)

Synergists teach ‘…no one knows the Father except those who choose the Son.’ But the Bible teaches that ‘no one knows the Father except the Son and anyone to whom the Son chooses to reveal Him’ (Matt 11:27) They are the ones who ‘choose’ the Son.

Synergists teach that ‘All can come to Christ of their own free will’, but Jesus teaches that ‘no one can come to me unless it is granted him by the Father.’ (John 6:65) and all whom He grants will come (John 6:37)

Synergists teach that ‘you are not Christ’s sheep because you do not believe’, but Jesus teaches that ‘you do not believe because you are not of My sheep.’ (John 10:26)

Synergists teach that ‘the reason you are not of God is because you are unwilling to hear and believe God’s words.’ Jesus, on the other hand, taught, ‘The reason why you do not hear [God’s words] is that you are not of God.” (John 8:47)

Synergists teach that ‘salvation is so easy a cave man can do it” but the Bible teaches that “What is impossible with man is possible with God.” (Luke 18:27)

In the Divine economy men are responsible to believe the gospel, but are morally impotent to do so from their own native resource. This inability (due to our intimate solidarity with Adam’s sin) is something we are culpable for, like owing a debt we cannot repay. So God has every right to call us all to account to ‘repay our debt’, so to speak, even though we do not have the resources to do so. The Church is to call all men to repent and believe the gospel (an imperative) but no one believes. But God, in his great mercy, still has mercy on many, opening their hearts to the gospel that that might believe.

To this sometimes a synergist often quotes “whosoever will may come” to which we reply that this quote does not teach an indicative of what we are able to do, but rather, teaches what we ‘ought’ to do. As Martin Luther said, “Does it follow from: ‘turn ye’ that therefore you can turn? Does it follow from “‘Love the Lord thy God with all thy heart’ (Deut 6.5) that therefore you can love with all your heart? What do arguments of this kind prove, but the ‘free-will’ does not need the grace of God, but can do all things by its own power…But it does not follow from this that man is converted by his own power, nor do the words say so; they simply say: “if thou wilt turn, telling man what he should do. When he knows it, and sees that he cannot do it, he will ask whence he may find ability to do it…” Luther BW,164