All those whom God hath predestinated unto life, and those only, he is pleased, in his appointed and accepted time, effectually to call, by his Word and Spirit, out of that state of sin and death in which they are by nature, to grace and salvation by Jesus Christ: enlightening their minds, spiritually and savingly, to understand the things of God, taking away their heart of stone, and giving unto them an heart of flesh; renewing their wills, and by his almighty power determining them to that which is good; and effectually drawing them to Jesus Christ; yet so as they come most freely, being made willing by his grace.

Westminster Confession of Faith, X, 1

Q. 31. What is effectual calling?
A. Effectual calling is the work of God’s Spirit, whereby, convincing us of our sin and misery, enlightening our minds in the knowledge of Christ, and renewing our wills, he doth persuade and enable us to embrace Jesus Christ, freely offered to us "in the gospel.

Shorter Catechism

Why do some believe in Jesus, and others do not? Does God make the difference? Or does man?

We've already looked at man's condition in original sin. Not a pretty picture. Can a dead person will himself to life?

Jesus answered him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.”

John 3:3

There is nothing therefore, against which Calvinism sets its face with more firmness than every form and degree of auto-soterism. Above everything else, it is determined to recognize God, in His Son Jesus Christ, acting through the Holy Spirit whom He has sent, as our veritable Saviour. To Calvinism, sinful man stands in need, not of inducements or assistance to save himself, but precisely of saving; and Jesus Christ has come not to advise, or urge, or woo, or help him to save himself, but to save him; to save him through the prevalent working on him of the Holy Spirit. This is the root of the Calvinistic soteriology, and it is because this deep sense of human helplessness and this profound consciousness of indebtedness for all that enters into salvation to the free grace of God is the root of its soteriology. that election becomes to Calvinism the cor cordis of the gospel. He who knows that it is God who has chosen him, and not he who has chosen God, and that he owes every step and stage of his salvation to the working out of this choice of God, would be an ingrate indeed if he gave not the whole glory of his salvation to the inexplicable election of the divine love.

B. B. Warfield
"The Theology of John Calvin"