It is a rerference to a passage from the Bible. In John 8:7, Jesus faced a mob that was eager to execute a woman caught in adultery. He put a stop to it with a simple challenge: anyone who is without sin should step forward and "cast the first stone." That sentence is often cited as a reminder to avoid judging others when there are faults in your own life that need to be addressed.

It is a reference to Bible Scripture where Jesus was admonishing a group of men over a woman who was caught in adultery (John 8:7). They brought her to him to test him! Jesus said to them: "he who is without sin, let him cast the first stone at her". It was a matter of judgement. With none qualified, they left without further incidence. The question then, was, "where are your accusers?"

"Пусть первым бросит в меня камень..."