Monergism

Irresistible Grace: The fourth tenet of Calvinism, the doctrine of irresistible grace teaches that all those for whom Christ died are those in whom the Holy Spirit works regeneration, and that the Holy Spirit's working is irresistible, that is no human being who is engaged by the Holy Spirit rejects His working.
Two-Age Glossary

The fact that calling is an act of God, and of God alone, should impress upon us the divine monergism in teh initiation of salvation in actual procession. We become partakers of redemption by an act of God that instates us in the realm of salvation, and all the corresponding changes in us and in our attitudes and reactions are the result of the saving forces at work within the realm into which, by God’s sovereign and efficacious act, we have been ushered. The call, as that by which the predestinating purpose begins to take effect, is in this respect of divine monergism after the pattern of predestination itself. It is of God and of God alone.
John Murray “The Call” from Collected Writings of John Murray (2:166)

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. In the Reformation view, the work of regeneration is performed by God and by him alone. The sinner is completely passive in receiving this action. Regeneration is an example of God's operative grace. Any cooperation we display toward God occurs only after the work of regeneration has been completed. Of course we respond to this work. We respond in a manner similar to that of Lazarus when, after being loosed, he stepped out of the tomb.
R.C. Sproul from What is Reformed Theology?