Romans 8:29-30 tells us that to those whom God has chosen for salvation, he has also given a purpose, namely, to conform to the likeness of his Son. And to those whom he has given such a purpose, he also issues a call to them in due time so that they may come to Christ. Thus the passage says, "Those he predestined, he also called" (v. 30).
Remember that all who are included in one phase of the application of redemption also enter into the next phase. All whom God has elected, he has also predestined, and all whom God has predestined, he also calls to Christ. But verse 30 goes on to say, "Those he called, he also justified." Thus all whom God calls will attain justification. And since justification is by faith in Christ, all whom God calls will believe in Christ and be justified. Therefore, God's calling toward the elect is bound to be effective, and so theologians call this act of God an EFFECTUAL CALLING.
Since the effectual calling is one whose result is guaranteed, it is not like an "invitation" that the elect may accept or reject. Rather, it is more like what we mean by the verb "to summon." In calling his elect, God does not merely invite them to do something, but God himself does something to them. Sinclair Ferguson writes, "He who calls them creates in them the ability to respond so that in the very act of his calling he brings them into new life." Thus those whom God has chosen and predestined in eternity, he also summons to Christ in historical time.
God summons the elect usually through the preaching of the gospel. Now, Christians do not first learn the identities of the elect, and then proceed to preach the gospel only to them. Rather, they preach the gospel "to all creation," and "Whoever believes and is baptized will be saved, but whoever does not believe will be condemned" (Mark 16:15-16). Therefore, whether it is in the form of public oration, private conversation, written literature, or other means, the preaching or presentation of the gospel goes forth to both the elect and the non-elect. The elect will come to faith; the non-elect will either reject the gospel, or produce a temporary and false profession of faith.
Because of this, theologians distinguish between the OUTWARD CALLING and the INWARD CALLING. The outward calling refers to the preaching of the gospel by human beings, and is presented to both the elect and the non-elect. On the other hand, the inward or effectual calling is a work of God accompanying the outward calling to cause the elect to come to faith in Christ. The preaching of the gospel appears to everyone as an outward calling, but it also comes as an inward summons to the elect. The outward calling is produced by human beings, but the inward calling is solely a work of God and occurs only to the elect. The inward calling is usually concurrent with the outward calling. In other words, many people may hear the gospel in a given situation, but God will cause only the elect to believe what is preached, while he hardens the non-elect against it.
Matthew 22:14 says, "For many are invited, but few are chosen." The word "invited" in this verse may be translated "called," as many other translations have it. Many are indeed "invited" in that they hear the outward call of the gospel, but only a few are among God's elect, and therefore genuine and permanent professions of faith only come from the latter group.
 Sinclair B. Ferguson, The Christian Life: A Doctrinal Introduction; Carlisle, Pennsylvania: The Banner of Truth Trust, 1997 (original: 1981); p. 34.