Irresistible grace, also known as efficacious grace, is the teaching that when God calls His elect to salvation through the Holy Spirit, that work is infallibly effective. Reformed traditions hold that while the human will is not violated, the Spirit so renews the heart that the sinner is made willing and is brought to faith in Christ.
The doctrine of irresistible, or efficacious, grace asserts that God's purpose in calling the elect is never frustrated. As described in the 1689 LBCF Ch.10 §1, God takes away the heart of stone and gives a heart of flesh, renewing the will so that the sinner comes to Christ freely, yet being made willing by His grace. This follows the same emphasis found in the Westminster Confession of Faith Ch.10 §1, which underscores that this calling is solely of God's power. It is a work of such transformative power that it is compared to the resurrection of Christ, as noted in 1689 LBCF Ch.10 §2, ensuring the sinner is enabled to embrace the grace offered.
Irresistible grace does not mean that God drags an unwilling person against their desires into heaven. Rather, it means that the Holy Spirit so illuminates the mind and softens the affections that Christ becomes the most beautiful and necessary treasure. As the Scripture says, Philippians 2:13 teaches us that it is God who works in us both to will and to work for his good pleasure. This grace humbles the sinner, removing any room for boasting, and ensures that the promise of Christ remains true: John 6:37 'All that which the Father giveth me shall come unto me; and him that cometh to me I will in no wise cast out.' We are more flawed than we ever dared believe, yet because this grace is rooted in Christ's finished work, we are more accepted and loved in Him than we ever dared hope.