Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 6. Christian Art” (Includes sections 75-81).
§ 77. The Cross and the Crucifix.
After another brief bibliography Schaff discusses the symbol of the cross. As early as the second century, Christians would make the sign of the cross (Schaff 2014, Loc. 15671) at various times throughout the day. It was used in baptism and is also commonly found on tombs and decorations. Schaff notes also the Χ and Ρ, often pulled together into one symbol, which represented Christ (Schaff 2014, Loc. 15677). The Χ, of course, can easily be altered into a cross shape. The A and Ω would also often be added, symbolic of Christ as the “first and the last.” This became very common in the fourth century after Constantine (Schaff 2014, Loc. 15682). Romans noted the cross as a symbol of shame, but Christians countered by observing it is also a symbol of triumph when used in a flag standard or a man or bird with outstretched arms or wings. The symbol is used both for triumph or a curse in numerous cultures (Schaff 2014, Loc. 15682).
Crucifixes with a representation of Jesus on the cross are found after the mid sixth century (Schaff 2014, Loc. 15688).