The following is an expression of the extraordinarily balanced understanding of John Calvin with regards to the work of the Holy Spirit in bringing persons to faith through the preaching of the gospel. Commenting on Acts, it shows the outworking of God's plan through the agency of men in casting forth the seed of the gospel, which, the Bible testifies, can only be responded to when germinated (so to speak) by the Holy Spirit. The Scripture testifies that that the word alone is not enough to enter and change the heart of natural man (who is hostile to God's word) but that the heart must be opened and the mind illumined by the concurrent work of the Spirit. An biblical example of this can be found in 1 Thes 1:4, 5. "For we know, brothers loved by God, that he has chosen you, 5because our gospel came to you not only in word, but also in power and in the Holy Spirit and with full conviction..."
by John Calvin
“One who heard us was a woman named Lydia, from the city of Thyatira, a seller of purple goods, who was a worshiper of God. The Lord opened her heart to pay attention to what was said by Paul.” Acts 16:14-15
Now when in fact only one hears attentively and effectively, could it not have appeared that the way was blocked for Christ to make an entry? But afterwards from that frail shoot a famous church sprang up, whose praises Paul sings in splendid terms. Yet it is possible that Lydia had some companions, of whom no mention is made, because she herself far surpassed them. Yet Luke does not attribute the cause for this one woman having shown herself docile, to the fact that she was sharperwitted than the others, or that she had some preparation by herself, but says that the Lord opened her heart, so that she gave heed to Paul’s words. He had just praised her piety; and yet he shows that she could not understand the teaching of the Gospel without the illumination of the Spirit. Accordingly we see that not only faith, but also all understanding of spiritual things, is a special gift of God, and that ministers do not accomplish anything by speaking, unless the inward calling of God is added at the same time.
By the word heart Scripture sometimes means the mind, as when Moses says (Deut. 29:4), ‘until now the Lord has not given you a heart to understand.’ So also in this verse Luke means not only that Lydia was moved by the inspiration of the Holy Spirit to embrace the Gospel with a feeling of the heart, but that her mind was illuminated to understand. We may learn from this that such is the dullness, such the blindness of men, that in hearing they do not hear, or seeing they do not see, until God forms new ears and new eyes for them.
But we must note the expression that the heart of Lydia was opened so that she paid attention to the external voice of a teacher. For as preaching on its own is nothing else but a dead letter, so, on the other hand, we must beware lest a false imagination, or the semblance of secret illumination, leads us away from the Word upon which faith depends, and on which it rests. For in order to increase the grace of the Spirit, many invent for themselves vague inspirations so that no use is left for the external Word. But the Scripture does not allow such a separation to be made, for it unites the ministry of men with the secret inspiration of the Spirit. If the mind of Lydia had not been opened, the preaching of Paul would have been mere words; yet God inspires her not only with the mere revelations but with reverence for His Word, so that the voice of a man, which otherwise would have vanished into thin air, penetrates a mind that has received the gift of heavenly light.
Therefore let us hear no more of the fanatics who make the excuse of the Spirit to reject external teaching. For we must preserve the balance which Luke established here, that we obtain nothing from the hearing of the Word alone, without the grace of the Spirit, and that the Spirit is conferred on us not that He may produce contempt of the Word, but rather to instill confidence in it in our minds and write it on our hearts.
1 From Calvin, John. The Acts of the Apostles. 2 vols. Trans. by John W. Fraser and W.J.G. McDonald. Ed. by David W. Torrance and Thomas F. Torrance. Grand Rapids, MI: Eerdmans, 1965.