What does “the Lord opened her heart” (Acts 16:14) mean, according to Calvinists?

Question

Acts 16:13 (NIV)
On the Sabbath we went outside the city gate to the river, where we expected to find a place of prayer. We sat down and began to speak to the women who had gathered there.

I conclude the situation is something like this:
Just before the writer sit down, there are already some women there. This situation I name it, time-frame-1. Time-frame-2 is after the apostles sat down then begin speaking to the women.

Acts 16:14a (NIV)
One of those listening was a woman from the city of Thyatira named Lydia, a dealer in purple cloth. She was a worshiper of God.

Next, I assume what the writer means (inspired by the Holy Spirit) in the bolded sentence is that Lydia already as a believer in the sight of God at time-frame-1 before time-frame-2.

Acts 16:14b (NIV)
The Lord opened her heart to respond to Paul's message.

The event "The Lord opened her heart" is either while Paul is talking (time-frame-2) or after Paul finished his talking. So, this event not happened at time-frame-1.

My question :
What does "The Lord opened her heart" mean according to Calvinists?

Answer 1

Calvinists consistently hold that Lydia, at the time Paul met her, followed the Jewish religion and was not a believer in Christ. But then God worked in her heart so that she would respond to Paul's teaching of the gospel.

At first, in "time frame 1," as you call it, Lydia "was a Jewess" (Calvin) or "a proselyte to the Jewish faith" (Jamieson, Fausset, and Brown). Whether she is Jew or Gentile is rather uncertain, says John Gill, as her name doesn't clearly indicate her ancestry. Either way, however, she worshiped "the true God, the God of Israel" (Gill).

However, Gill continues, that even in this state, her heart "was shut and barred, with the bars of ignorance, hardness, and unbelief." Calvin notes that Luke "commended her godliness" yet shows "that she could not comprehend the doctrine of the gospel, save only through the illumination of the Spirit."

Thus Calvinists argue that she was not saved in "time frame 1," as you suggest, and instead that in "time frame 2," "the Lord opens [her] heart to hear the word which is preached" (Geneva Study Bible). This passage is seen as evidence for the distinction between the "inward call" of God and the "external call" of the preacher. Calvin writes:

We see that not faith alone, but all understanding and knowledge of spiritual things, is the peculiar gift of God, and that the ministers do no good by teaching and speaking unless the inward calling of God be thereunto added.

And in reference to Lydia in verse 14, the Reformation Study Bible (2015) states:

Divine illumination and persuasion is necessary for the heart blinded by sin to respond to the gospel. This effectual call of God ensures that all who have been chosen by God will believe.

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• Calvin on Acts 16:11–15
• Geneva Study Bible on Acts 16
• Gill on Acts 16
• Jamieson, Fausset, and Brown on Acts 16