How did pagan idolaters in Paul's cities become believers willing to be martyred?
They became believers by the power of God when the Gospel was preached. Into morally corrupt pagan cities, Paul preached Christ crucified, and those people turned from their idols, becoming so staunch in faith that they were willing to be martyred by the hundreds without wavering.
Les explains that Thessalonica, Berea, Athens, and Corinth were abject pagan idolaters who knew nothing of the God of creation and were morally rotten. Into that environment Paul walked with no pleasant appearance and nothing physically going for him, and all he did was preach Christ crucified. Those people turned from their idols and became believers, staunch in faith, willing to be martyred by the hundreds; they never wavered, did not give up, panic, or run. The explanation is the power of God when the Gospel was preached, and Les thinks it is the same way today.