Many have wondered what it was that Christ wrote in the sand/dust when the woman was brought by the Pharisees to be stoned for adultery. The passage from St John, telling of this event, is the only one in the Bible where there is mention of Christ ever having written anything. Knowing the human tendency to make idols out of words and sacred books, Christ probably decided not to personally write any scriptures (neither did Muhammad nor the Buddha).
The Bible doesn't say what Christ wrote, and so there has been a lot of theological conjecture about it. The most interesting explanation is that He was writing down the sins of those who had gathered to carry out the stoning. In Jewish tradition, when an adulterer was brought to the Temple for punishment, their sin was written in the dust of the court floor and then brushed away (perhaps because the sin was considered too unmentionable to say aloud in such a holy place), but the Pharisees seem to have neglected to do this. They were also supposed to have brought for punishment the man caught in adultery, not just the woman. So, though they were presenting themselves as upholders of the law, they didn't even follow the letter of the law, let alone the spirit of the law, which is no doubt what Christ was trying to show them.
By writing down their hidden sins (as tradition stated the adulterer's sin should be written) Christ showed the Pharisees that He thoroughly knew the law. And when they saw their sins written on the earth, they were too shocked to carry out the stoning. John says they walked away one by one, leaving the woman with Jesus, who told her that no one will condemn her, including Himself.
Appealing to their consciences, by just telling the Pharisees they were hypocrites ("he who is without sin may cast the first stone") would not have been enough for such people. They had to see their sins written for all to read.
Knowing He preached forgiveness, the Pharisees had tried to trick Christ into publicly going against the Old Testament laws, so they could accuse Him of blasphemy. Only a divine personality could have resolved the situation so perfectly.
Jesus and the woman taken in adultery, by Julius Schnorr von Karolsfeld, 1860.