Even when we were dead in our trespasses, [God] made us alive together with Christ — by grace you have been saved. (Ephesians 2:5)
The great missionary hope is that when the gospel is preached in the power of the Holy Spirit, God himself does what man cannot do: he creates the faith that saves. The call of God does what the call of man can’t. It raises the dead. It creates spiritual life. It is like the call of Jesus to Lazarus in the tomb, “Come out!” And the dead man obeyed and came out. The call created the obedience by creating life (John 11:43). That is how anyone is saved.
We can waken someone from sleep with our call, but God’s call can summon into being things that are not (Romans 4:17). God’s call is irresistible in the sense that it can overcome all resistance. It is infallibly effective according to God’s purpose — so much so that Paul can say, “Those whom [God] called he also justified” (Romans 8:30), even though we are only justified by our faith.
In other words, God’s call is so effectual that it infallibly creates the faith through which a person is justified. All the called are justified according to Romans 8:30. But none is justified without faith (Romans 5:1). So the call of God cannot fail in its intended effect. It irresistibly brings into being the faith that justifies.
This is what man cannot do. It is impossible. Only God can take out the heart of stone (Ezekiel 36:26). Only God can draw people to the Son (John 6:44, 65). Only God can open the spiritually dead heart so that it gives heed to the gospel (Acts 16:14). Only the Good Shepherd knows his sheep, and calls them by name with such compelling power that they all follow — and never perish (John 10:3–4, 14).
The sovereign grace of God, doing the humanly impossible, through the gospel of Jesus Christ, is the great missionary hope.