Read Acts 16:11-15
Lydia has all the hallmarks of a seeker: she was already interested in spiritual things, she had some knowledge of God, was a worshipper of God, presumably had (at least) some knowledge of the bible and was looking for more. But the bible does not say “she opened her heart” - rather it declares that “God opened” it.
If Lydia can't make it on her own, who can? The answer is no-one. No-one will ever become a Christian without the intervention of God. We simply don't have the power to open people's hearts – only God does.
Think of the non-Christians you know: how often do you pray for them? After all – prayer is how we call on God to act and to save.
But what means does God use to open her heart? He could have used a vision, or a dream or writing in the sky. But it didn't: God chose the moment that Paul was sharing the gospel with her. Having prayed – and trusting God to come through – get ready to be bold, step out in faith. Because God's top choice for opening people's hearts to the gospel is the point in time that someone is telling them about Jesus.