The Symbol of the Cross 

History shows that the cross was used centuries before Christ.
"From its simplicity of form, the cross has been used both as a religious symbol and as an ornament, from the dawn of man's civilization.
Various objects, dating from periods long anterior to the Christian era, have been found, marked with crosses of different designs, in almost every part of the old world.
India, Syria, Persia and Egypt have all yielded numberless examples, while numerous instances, dating from the later Stone Age to Christian times, have been found in nearly every part of Europe.
The use of the cross as a religious symbol in pre-Christian times, and among non-Christian peoples, may probably be regarded as almost universal, and in very many cases it was connected with some form of nature worship".
(The Encyclopedia Britannica, 11th ed., 1910, Vol. 7, pg. 506. Emphasis ours.)

The surprising thing is that the Christian use of the cross did not begin until the time of Constantine, three centuries after Christ.
Archaeologists have not found any Christian use of the symbol before that time.
According to one writer (W. E. Vine, Expository Dictionary of New Testament Words, article "Cross"), the cross as a "Christian" symbol was taken directly from the pagans.

The New Testament does not specifically describe the instrument upon which Christ died, though Acts 5:30; 10:39; and 13:28-29 refer to it as a "tree."
The Greek word xulon, translated "tree" in these verses, can mean a stick, club, tree, stake, or other wooden articles.
There is absolutely no evidence that God's true church ever used the cross symbol for any purpose.
Nowhere does the Bible command its use however, throughout the world, people universally regard the cross as THE symbol of Christianity.
Among Christians it recalls the crucifixion of Jesus and humanity's redemption thereby.
The Christian form of blessing by tracing a cross over oneself or another person or thing.

This cross is the reflection of a light on a wall in the Église de Saint-Eustache which is a church in the Ier arrondissement of Paris, built between 1532 and 1632.
It was shot during the midnight Mass celebrated on Christmas Eve.