The cross

Staurogramm, Tau Rho

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Wikipedia tells

The cross, which is today one of the most widely recognised symbols in the world, was used as a symbol from the earliest times. This is indicated in the anti-Christian arguments cited in the Octavius of Minucius Felix, chapters IX and XXIX, written at the end of the 2. century (cited as AD 197) or the beginning of the next.

By the early 3rd century the cross had become so closely associated with Christ that Clement of Alexandria, who died between 211 and 216, could without fear of ambiguity use the phrase τὸ κυριακὸν σημεῖον (the Lord's sign) to mean the cross, when he repeated the idea, current as early as the Epistle of Barnabas, that the number 318 (in Greek numerals, ΤΙΗ) in Genesis 14:14 was a foreshadowing (a "type") of the cross (T, an upright with crossbar, standing for 300) and of Jesus (ΙΗ, the first two letters of his name ΙΗΣΟΥΣ, standing for 18).

His contemporary Tertullian could designate the body of Christian believers as crucis religiosi, i.e. "devotees of the Cross". In his book De Corona, written in 204, Tertullian tells how it was already a tradition for Christians to trace repeatedly on their foreheads the sign of the cross.

The Jewish Encyclopedia states:

The cross as a Christian symbol or "seal" came into use at least as early as the second century (see "Apost. Const." iii. 17; Epistle of Barnabas, xi.-xii.; Justin, "Apologia," i. 55-60; "Dial. cum Tryph." 85-97); and the marking of a cross upon the forehead and the chest was regarded as a talisman against the powers of demons (Tertullian, "De Corona," iii.; Cyprian, "Testimonies," xi. 21-22; Lactantius, "Divinæ Institutiones," iv. 27, and elsewhere). Accordingly the Christian Fathers had to defend themselves, as early as the second century, against the charge of being worshipers of the cross, as may be learned from Tertullian, "Apologia," xii., xvii., and Minucius Felix, "Octavius," xxix. Christians used to swear by the power of the cross.
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There are thus literal references to cross as a Christian sign in early 3d century literature. However, no 3rd century or earlier images of the cross have been found.

During the first two centuries of Christianity, the cross may have been rare in Christian iconography, as it depicts a purposely painful and gruesome method of public execution and Christians were reluctant to use it. A symbol similar to the cross, the staurogram, was used to abbreviate the Greek word for cross in very early New Testament manuscripts such as P66, P45 and P75, almost like a nomina sacra. The extensive adoption of the cross as Christian iconographic symbol arose from the 4th century.
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We may suggest that as Emperor Constantine the Great forbid the use of crucifixion the horror and shame associated with this method of execution of rebels and slaves was forgotten during the 4th century and the use of the cross symbol began.