Cross in Early Christianity

The most recognizable and important Christian symbol is the cross. But when did Christians begin to use the cross as their significant symbol? I ask the question because in my ten days in Rome visiting the sites where it is possible to see early Christian art, not once did I see a cross. Not in the catacombs, not in the Vatican Museum collection of early Christian artifacts, not in the ancient city of Ostia where there is artwork on the floor of a house - chalice and fish - that might have been Christian, not in the early artwork in the church Giovanno e Paulo, and not in the artifacts used to build the foundation of the Basilica di Santa Maria in Trestevere. At all of these sights it is possible to see other early Christian symbology: the fish, the anchor, the orante, bread and cup, and more, but no crosses anywhere.

It would be tempting to conclude that the cross was not an important symbol yet. And I have heard it said that the cross really wasn't an important symbol until after the conversion of Constantine, who conquered under the sign of the cross, and forever changed the relationship of Christianity to the state.

But it isn't quite that simple, it seems. I am reading Robert Grant's book Greek Apologists of the Second Century, and in a discussion of the writings of the apologist Justin "the Martyr" he notes that Justin "defends" the cross of Christ by arguing that the cross occurs in nature, scripture, and in human society. So somebody was using the cross, apparently. And Justin's writing would have been contemporaneous with some of the art and artifacts I was seeing in Rome. So how prevalent was its use in art, writings, and in the minds of early Christians?

This is the kind of thing that I wake up wondering about in the morning.