Answer: Effectual calling is the work of God’s almighty power and grace₁, whereby (out of his free and special love to his elect, and from nothing in them moving him thereunto₂) he does, in his accepted time, invite and draw them to Jesus Christ, by his Word and Spirit₃; savingly enlightening their minds₄, renewing and powerfully determining their wills₅, so as they (although in themselves dead in sin) are hereby made willing and able freely to answer his call, and to accept and embrace the grace offered and conveyed therein.₆

1. John 5:25; Ephesians 1:18-20; 2 Timothy 1:8-9
2. Titus 3:4-5; Ephesians 2:4-5; Ephesians 2:7-9; Romans 9:11
3. 2 Corinthians 5:20; 2 Corinthians 6:1-2; John 6:44; 2 Thessalonians 2:13-14
4. Acts 26:18; 1 Corinthians 2:10; 1 Corinthians 2:12
5. Ezekiel 11:19; Ezekiel 36:26-27; John 6:45
6. Ephesians 2:5; Philippians 2:13; Deuteronomy 30:6

For further study see:

1. WSC 30: How doth the Spirit apply to us the redemption purchased by Christ?
2. Q30: How doth the Spirit apply to us the redemption purchased by Christ?
3. WCF CHAPTER 10: Of Effectual Calling
4. Dr. R.C. Sproul explains “effectual calling” which is also known as “irresistible grace”: