It just sounded so fake and so contrived, so I did a little research about it.  Someone on here said it sounded like a scene from Ben-Hur, so I did a google search about Ben-Hur and cross in the sand and such.  No dice.  But I searched around a little bit more and here is what I found.  A story about Alexander Solzhenitsyn from his times in the Soviet Gulags.

Along with other prisoners, he worked in the fields day after day, in rain and sun, during summer and winter. His life appeared to be nothing more than backbreaking labor and slow starvation. The intense suffering reduced him to a state of despair.

On one particular day, the hopelessness of his situation became too much for him. He saw no reason to continue his struggle, no reason to keep on living. His life made no difference in the world. So he gave up.

Leaving his shovel on the ground, he slowly walked to a crude bench and sat down. He knew that at any moment a guard would order him to stand up, and when he failed to respond, the guard would beat him to death, probably with his own shovel. He had seen it happen to other prisoners.

As he waited, head down, he felt a presence. Slowly he looked up and saw a skinny old prisoner squat down beside him. The man said nothing. Instead, he used a stick to trace in the dirt the sign of the Cross. The man then got back up and returned to his work.

As Solzhenitsyn stared at the Cross drawn in the dirt his entire perspective changed. He knew he was only one man against the all-powerful Soviet empire. Yet he knew there was something greater than the evil he saw in the prison camp, something greater than the Soviet Union.  He knew that hope for all people was represented by that simple Cross. Through the power of the Cross, anything was possible.

Solzhenitsyn slowly rose to his feet, picked up his shovel, and went back to work. Outwardly, nothing had changed. Inside, he had received hope.

[From Luke Veronis, "The Sign of the Cross"; Communion, issue 8, Pascha 1997.]

So, it is very interesting that Mr. Solzhenitsyn and Mr. McCain had the same Christian guard experience.  Maybe the Soviet guard and the Vietcong guard were trained by the same Christian/Communist person.  Or maybe it is all just a made up story.  Somehow I doubt that Alexander Solzhenitsyn heard John McCain's story and copied it.

UPDATE:  This story was actually excerpted from "The Gulag Archipelago" by Alexander Solzhenitsyn, which was definitely written before John McCain's books.