How do we know that God opens hearts to receive the Gospel?

In the narrative of Lydia within Acts 16, we learn of the divine activity of God in opening people's hearts. When Paul preached, it was not through human eloquence but through the divine operation of the Holy Spirit that Lydia's heart was opened. This reinforces the Reformed belief in the 'effectual calling' of the Holy Spirit, where certain individuals are chosen by God to hear and embrace the gospel. The phrase 'whose heart the Lord opened' succinctly captures this truth, showing that human beings can do nothing to convert themselves without God's divine intervention. This can further be supported by passages such as Romans 8:30, which explains that those whom God predestines are also called and ultimately justified, highlighting the sovereignty of God in the process of salvation.