وَلَقَدْ فَتَنَّا الَّذِينَ مِن قَبْلِهِمْ ۖ فَلَيَعْلَمَنَّ اللَّهُ الَّذِينَ صَدَقُوا وَلَيَعْلَمَنَّ الْكَاذِبِينَ
“We certainly tested those before them. And [in this way] Allah will clearly distinguish between those who are truthful and those who are liars.”
Cited in the lecture to explain why the early believers were tested through persecution.