Who opened Lydia's heart so that she paid attention to Paul's preaching?

Les teaches that the verse plainly says, "whose heart the Lord opened," and he urges his listeners to underline or highlight it. The Lord opened it. Les points out that even the great apostle Paul, with all the power he had, could not open the hearts of those people with his preaching — it took the work of the Holy Spirit to open Lydia's heart, and then the words of Paul took root. Les says it is no different today: you and I can do nothing, and we can never get across any of these truths unless the Holy Spirit opens the heart and spiritual eyes of those to whom we speak. He compares his own words to nothing more than a tinkling cymbal unless God Himself opens hearts, and notes that many have asked why they did not catch this fifteen years earlier — it simply was not God's time, but in His own time He opens hearts and spiritual understanding.