This rare compound describes God's unique ability to know the human heart intimately. In Acts 1:24, the apostles pray for guidance in choosing Judas's replacement: 'Lord, you who know the hearts (καρδιογνώστης) of all, show us which one of these two you have chosen.' The appeal to God as heart-knower grounds their petition—only God can see past external qualifications to the true state of a person's soul. In Acts 15:8, Peter declares that 'God, who knows the heart (καρδιογνώστης), bore witness to them [Gentiles] by giving them the Holy Spirit just as he did to us,' making divine heart-knowledge the basis for accepting Gentile believers without requiring circumcision.