Definitions of Effectual Calliing:
"this calling is that act by which those, who are chosen by God and redeemed by Christ, both externally and internally are sweetly invited and effectually brought from a state of sin to a state of communion with God." (Herman Witsius, The Economy of the Covenants, Vol. 1, pg. 344)
" Effectual calling is the sovereign summons of the triune God, by which, according to his eternal election of individuals, through undeserved grace he produces gospel faith and repentance in them to create a new people in spiritual union with Christ, who live in holiness while God infallibly brings them to glory." (Joel R. Beeke)
The Greek word kaleo ("to call") can refer to an invitation (Luke 14:13), but can also bear the more forceful sense of a "summons" (Isaiah 65:12; Matthew 4:19-22; 22:1-3).
Scripture uses the term "call" to refer to when God, through His providence, summons people to do His will (Isaiah 40:26; Jeremiah 1:15; Ezekiel 38:21; Note: Jeremiah uses the Hebrew term qara in 1:15, which literally means "to call".)
Biblical Evidence for Effectual Calling
A key distinction within Reformed theology on this point that is necessary to understand is between the vocatio interna (internal call) and the vocatio externa (external call). Arminians claim that Scripture does not present such a distinction at all, and hence that whenever Scripture speaks of people being "called" to salvation, it is in reference only to the external call and invitation of the Gospel that gives through evangelism and preaching.
The best passage to illustrate that there is such a thing as an effectual, internal call of God to salvation is Romans 8:30, which says "And those whom he predestined he also called, and those whom he called he also justified..." If "called" here referred to the external call of the gospel proclamation, then it would mean that everyone who is externally invited here has also been previously predestined by God (the text teaches clearly that everyone who has been predestined is also called.) If the "calling" here is in reference to "external call", why are there so many people under the preaching of the Gospel who are still hard-hearted and reject the Gospel? Thus, in order to make any rational sense of this text, we must conclude that it speaks of an internal call of God which takes place within the souls of the elect people of God.
Here are some biblical examples and evidences of the doctrine of effectual calling:
"And you also are among those who are called to belong to Jesus Christ. To all in Rome who are loved by God and called to be saints" (Romans 1:6-7)
"To the church of God that is in Corinth, to those sanctified in Christ Jesus, called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ.....God is faithful, by whom you were called into the fellowship of his Son, Jesus Christ our Lord." (1 Corinthians 1:2, 9)
"But when he who had set me apart before I was born, and who called me by his grace," (Galatians 1:15)
"who saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace, which was granted to us in Christ Jesus from all eternity," (2 Timothy 1:9)
Effectual Calling creates a new people (Isaiah 48:13; John 5:25)
Effectual Calling produces faith and repentance in the elect (1 Corinthians 12:3)
A Note on the External Gospel Call/Invitation
Many have caricatured Calvinism as being inept for evangelistic purposes, saying that Calvinists cannot in any well-meaning way, invite people to Christ. I would respond by saying that this is hyper-Calvinism, rather than traditional Calvinism. Note what the following paragraph from the Canons of Dort says:
"Moreover, the promise of the gospel is, that whosoever believeth in Christ crucified, shall not perish, but have everlasting life. This promise, together with the command to repent and believe, ought to be declared and published to all nations, and to all persons promiscuously and without distinction, to whom God out of His good pleasure sends the gospel." (Canons of Dort, Second Head of Doctrine, Article 5)