What was the writing in the dust?

It was GEOMANCY. Geomantic inscriptions. Jesus was strategically calling them out on their wicked DIVINATION black magic. It was silently showing the self-righteous that the True Messiah from Father God KNEW EXACTLY how the accusers (Pharisees and Religious Lawyers) were GUILTY and their secret sins, and that God's law demanded they TOO be stoned to death, alongside the adulteress.

In divine wisdom, Jesus simultaneously:
(a) demonstrated he indeed was the Messiah, and knew their innermost secrets and darkest sins already,
(b) upheld God's law given to Moses, and in admitting that she was indeed guilty of death by stoning, he also ingeniously INCLUDED ALL HER ACCUSERS as even more guilty of death by stoning
(c) presented Himself as Messiah - the Son of God who had power to forgive sin, but not accept sin, thereby extending grace and mercy and a way to have LIFE and avoid being condemned to death.

Jesus is FURIOUS at the deception and hypocrisy of these self-appointed spiritual leaders who were in fact black magicians. Jesus BEATS EVIL at it own game. Jesus TURNS THE TABLES on accusation, because as Messiah he invites us into forgiveness and life that only He, as God's Son, has the credentials, righteous sinless standing and authority to offer us.

This is God's love and grace at work, demonstrating full awareness of the occultists in Judaism that pretending to be pious leaders. Jesus is our marvelous hero, our savior, our answer to accusation, our escape from death to Divine Life.