The sign of the cross, both in symbology and also the gesture of ‘crossing oneself’ has it’s roots in Christianity and is recognised worldwide. It is chosen as it represents the crucifixion of Christ, the well known narrative from the Bible.
The cross represents the power of Jesus, who is said to have died to cleanse the sins of mankind. This is worn as a reminder of that sacrifice. The gesture, where the sign of a cross is made with the hands over the body is used as a form of prayer.