One hand raised is a gesture that Italians use to mean a wife has been cheating on a husband

All body language gestures signal something in the context in which they are used. The same is true of the spoken language. The word err means to make a mistake.

But if we use er as a pause filler—”I was—er—going to the—er—store”—as many of us do, the listener knows what we mean. He doesn’t confuse er with err, even though they are pronounced the same way.

The same is true of hand postures. They can signal one thing in a certain context and something else in a different context. The finger posture you noticed in the mosaic of St. Luke is called, in Latin, the Manu Cornuta, and it was commonly used to imply that a man was a cuckold —at least up until the seventeenth century. Now its use is rare and mixed. Sometimes it is an insult, and sometimes
it is a gesture of protection.

To make the Manu Cornuta, the two middle fingers and the thumb are tucked into the palm. The little finger and the index finger are pointed as if they were horns.

Originally, this was a pre-Christian gesture—in fact, a carved hand in this pose was found in the ruins of Herculaneum, the Italian city buried by Mount Vesuvius.
Later, the early Christians used the sign as a protection against the evil eye and generally to banish evil. This was probably how it was meant in the Italian mosaic
you saw.

Another hand gesture with historical significance is the Manu Fantea, in which all the fingers except the index and the one next to it are closed. These two are
extended together or in a V angle. This sign had a special significance in early Greek and Roman times. It was used in tombs and carvings to mean a wise man, a judge, or a teacher. Later it became the benediction sign of Christianity. It signaled V for victory in World War II, and then in the sixties, it became the peace sign.

There are fascinating historical backgrounds to various other hand postures, and, in an article called “The Communicative Hand,” Dr. J. A. V. Bates of the National Hospital for Nervous Disease in London traces many of them. In a very original bit of research, he follows the history of a little-studied gesture he calls the 101. It’s a hand with all fingers extended and spread except for the two middle ones, between the index and the little finger. These two are held touching. Try it. It’s a difficult an unnatural gesture.

Dr. Bates finds it first in church mosaics made in the tenth century in Palermo. 

One of the fathers of the church, St. Basil, holds his hands in this position, and so does the Virgin Mary in many of her portraits. In many crucifixes, it can be seen in the hand of Christ.

It disappears after the thirteenth century and appears again in the fifteenth in the hands of the baby Jesus and the Virgin in paintings from the Netherlands.

Evidently, it has been used all this time as a godlike gesture.

In the work of later painters, Botticelli and Bellini, the hand in this position began to appear on saints, and later Raphael used it in lesser characters in paintings in the Sistine Chapel.

It was then picked up in portraits. Titian used it for the repentant Mary Magdalene, and Bronzini gave it to hands in many paintings of men and women. Abbate, of the Fountainbleau school, gave the 101 hand posture to a brothel madam and a lecherous client of the brothel.

The signal had lost all its holy, godlike meaning. Today, Bates points out, he has seen it on a mannequin in the window of a Carnaby Street clothing store and in a poster for the play Jesus Christ Superstar, where Christ uses it to hold a microphone. Has it come full circle?