A friend once challenged me to read through the Bible with a highlighter. He said, “Everywhere you see something in the Bible about evangelism or sharing the gospel with the nations, highlight that.” Then he quipped, “If you do this, you’ll run out of highlighter before you run out of Bible.”
He’s right. The Bible is thoroughly focused on missionary and evangelistic endeavors. But this makes total sense because we know the Bible is fundamentally and primarily about God. It is about the glory of God being proclaimed through all of creation. It is about how sin has robbed God of that glory, but how God in his mercy is redeeming that which is lost so that sinners may be brought back into the glory of his holy presence. The Bible is fundamentally and primarily a book of God’s glory displayed in the Triune work of God giving God through God that we might be made sons of God. So, by definition, God’s grace in giving himself is an outward focused evangelistic event.
Theologians have described this outward expression of how God brings the blessing of God to sinners as “the calling.” Wilhelmus À Brakel defines the calling as, “a gracious work of God, whereby He invites the sinner by means of the gospel to exchange the state of sin and wrath for Christ, in order that through Him he may be reconciled to God and obtain godliness and salvation. By means of this calling He also, by the Holy Spirit, efficaciously translates His elect into this state.” When the Bible speaks about evangelism, it is speaking of this calling.
There are two different aspects to this calling that should be discussed. There is an external and an internal call to the gospel. Both come from God. Both occur by means of God’s Word. Both are presented to human beings who are by nature sinners. But there are key differences. This distinction is not explicitly termed in Scripture, but is easily deduced. Herman Bavinck explains with five reasons. First, not all people respond the same way to the calling. All are sinners (Rom. 3:9-19; 5:12; 9:21; 11:32). All are dead in their trespasses (Eph. 2:1-3). They are darkened in their understanding (1 Cor. 2:14; Eph. 4:18; 5:8). On their own they cannot do good (John 15:5; 2 Cor. 3:5). So, the different response among people requires there to be at least dual aspects to the calling. Otherwise, either all would be saved or none would. Second, simply the preaching of God’s Word is not enough. The Old Testament bears witness to the work of the Holy Spirit in regeneration (Isa. 32:15; Jer. 31:33; 32:39; Ezek. 11:19; 36:26; Joel 2:28). As does the New Testament (John 15:26-27; Acts 2:1-4). Third, the work of redemption belongs to God. It depends not on human will or exertion, but on God (Rom. 9:16). It is a divine work, so the calling must not only be the Word but the Spirit. Fourth, what is wrought in the heart by the Spirit is too great to be explained as an intellectual convincing by the preaching of the Word. Fifth, Scripture speaks of the calling in a dual sense. It speaks of a calling and invitation in which there is no positive response (Isa. 65:12; Matt 22:3, 14; 23:37). The gospel is proclaimed and yet some people remain in their obstinacy, “to one a fragrance from death to death” (2 Cor. 2:16). But others hear and believe, “to the other a fragrance from life to life” (2 Cor 5:16).
The explanation for the different responses to the call of God is that there is an external and an internal call. The external call happens by means of the Word. It is hearing the apostolic message concerning the Christ. It may result in a knowledge of God, even a true knowledge about God. But it does not necessarily result in saving faith. The internal call, however, is the work of the Holy Spirit who, in conjunction with and by means of the Word of God, operates upon the heart of man. His eyes are opened (Eph 1:18). His will is turned to Christ (Phil. 2:13). He is made alive (Eph 2:5-6). He is brought out of darkness and into his marvelous light (1 Pet. 2:9). It brings true knowledge of the apostolic message concerning the Christ together with personal trust in the same Christ. Both the external and internal are necessary for salvation.
God’s glory is displayed through his grace by giving himself for sinners. Because of this all Christians are to participate in the proclamation of this message. There is a universal offer of the grace of God. The external call is to be broadcast universally even if only some will respond with an internal call. The Scriptures are clear that the gospel is to be preached to all people. God will save those whom he will save. That is not our prerogative. It is to come to all people without distinction. We do not know whether one will respond in faith. We only know that the outcome is determined by God’s perfect will. But because we trust in God’s perfect will, we know that the preaching of the gospel will bear fruit. It will neither be ineffective nor useless. It will always accomplish its purpose (Isa. 55:11). And this purpose is not primary about man’s salvation but the glory of his name.
 Wilhelmus Brakel, The Christian’s Reasonable Service, 4 Vols. (Grand Rapids: Reformation Heritage Books, 2015), II.192.
 Herman Bavinck, Reformed Dogmatics (Baker Academic, 2008), 4.43.