I’ve always thought that what Jesus wrote on the ground was the verse Deuteronomy 22:24.
you must take both of them out to the gate of that city and stone them to death—the young woman because she did not cry out in the city, and the man because he has violated his neighbor’s wife. So you must purge the evil from among you.
That’s certainly possible. From the outcome, however—assuming it was Scripture He wrote—perhaps a better possibility might be, “I desire mercy, not sacrifice” or something similar.
Regardless, God chose not to reveal to us what Jesus wrote. Instead, all we have is the spectacle of Jesus taking time for due consideration of some type before He addressed the crowd. Indeed, who can doubt that the time he thus absorbed was designed to calm the passions of the rockholders?
So, that calm, deliberative, reasoned process remains our example.