Although the cross was used as a symbol long before the Christian era, it is the primary symbol of the Christian religion and is a symbol both of Christ himself and of the faith of Christians.
In ceremonial usage, making the sign of the cross may be an act of profession of faith, a prayer, or a dedication.
Although there are several different forms of the cross, tradition favors the Latin Cross (with one short and one longer arm set a right angles to each other) as that on which Christ died. You can see some of my designs reflecting the cross in the Cherish Collection.
Before the time of the emperor Constantine in the 4th century, Christians were hesitant to display the cross because it could expose them to danger.
After Constantine converted to Christianity he promoted the cross, which became immensely popular in Christian art and funerary monuments from c. 350.