I will say once again this not an internal work, not a cleansing and certainly not regeneration but rather if you pay attention to the fact that we need to be respectful of idiomatic expressions in scripture where this means Lydia was persuaded to pay attention because she was already someone who was already wanting to learn more.
So many assumptions made by the Reformed soteriology regrading this passage ....
1)
assume this heart opening is a doctrinal requirement for salvation
2)
assume the Lord exclusively uses supernatural heart-opening measures and does not use common ordinary circumstantial means to open someone's heart
3)
assume Lydia was lost or unsaved
4)
assume the things spoken of by Paul were the Gospel
5)
assume believing on Christ requires something additional to hearing see Romans 10: 14-17
6)
assume everyone else who is saved
must have their heart open in the same manner, thereby not taking into context that this was unique time in church history where God was intervening to help grow the ekklesia.
7)
assume this means an internal work
8)
assume this is circumcision of the the heart.
and one and on...one assumption after another.
Romans 10:14-17
14 How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard?
3 And how are they to hear without someone preaching?
15 And how are they to preach unless they are sent? As it is written,
w“How beautiful are the feet of those who preach the good news!”
16 But they have not all obeyed the gospel. For Isaiah says, “Lord, who has believed what he has heard from us?”
17 So faith comes from hearing, and hearing through the word of Christ.