The sign of the cross is one of the oldest Christian gestures still in use. Tertullian, writing around the year 200, describes Christians tracing the cross on their foreheads at every turn of the day — rising, dressing, eating, beginning any task. It was, from early on, simply what Christians did.
The fuller gesture — forehead, breast, and the two shoulders — developed over time, and the traditions make it differently: the Eastern Orthodox right to left, most of the Western churches left to right. The words that accompany it name the Trinity into which the Christian was baptized.
To make the sign is a small, wordless confession. It marks the body with the cross, recalls one's baptism, and claims the protection of the crucified and risen Lord. It is used across the Roman Catholic, Eastern Orthodox, Anglican, and Lutheran traditions, and by a growing number of other Christians who have rediscovered it.