The cross, with reference to Christ's passion event, is not found in Christian art in the first centuries.
"Death by crucifixion was infinitely more painful and degrading than is hanging or electrocution. During the first Christian centuries, the cross was a thing accursed. No one professed allegiance to Christ by wearing a cross. From personal witness everyone was fully aware of what death on a cross meant in terms of atrocious, long, drawn-out suffering. The cross was not something to be contemplated with equanimity. It provoked shuddering horror. The attitude of Christians to the cross changed in the fourth century, due to a number of factors. [...] Out of respect for the way Christ died, Constantine forbade crucifixion as a means of execution. Consequently, people began to forget precisely what was involved in this horrible form of murderous torture. It was no longer part of their experience. [...] A new, more pleasant meaning for the cross was facilitated by the discovery of the True Cross during the construction of the Church of the Holy Sepulchre in Jerusalem. The emphasis that it was made of wood transformed it into a positive symbol of the tree of life. [...] Even after the cross had been widely accepted as a symbol, there was a consistent refusal to accept its reality. Only two crucifixion scenes survive from the fifth century. On an ivory casket in the British Museum, London, a beardless Christ with arms at right angles seems to hover on the cross. On one of the doors of the Church of Santa Sabina in Rome, the arms of Christ are bent at the elbow (the posture of prayer) and nailed to the architectural background; no crosses are visible. There is no hint that the crucified figures are dying in agony. The situation remains unchanged until the twelfth century.
"During the first three centuries after Christ's death, the crucifixion was not shown in Christian art; the early church avoided the subject. Finally, in the fourth century, when Christianity was a proscribed religion under the Romans, the crucifixion was portrayed symbolically with the Lamb of Christ contiguous with Christ. Also, it was shown symbolically in the fourth century by a cross with no body. Christ is first shown on the cross in the early fifth century, attached by four nails in his hands and feet (Renaissance artists used three) and naked except for a loin-cloth, but he shows no trace of suffering.
"It may seem strange therefore that in the earliest Christian art the cross was not depicted realistically. [...] The cross which first appeared upon the monuments was the triumphal cross of Constantine, often in the form of the monogram he beheld in his vision. In the pagan symbols which resembled the cross Christians were inclined to see a presage of the Gospel. But the only symbol of this sort which they used before the fourth century was the swastica, an ancient pagan representation of the four winds, which had come to be little more than a decorative motif. [...] To the Egyptians it symbolized life, and Christians regarded the cross as the tree of life.
"Though it did not appear in pre-Constantinian art, (the cross) surfaced as an important Christian image after Constantine's victory in 312 and the freedom given to the Christian church. [...] Although the fourth century witnessed a gradual shift in Christian iconography away from biblical narratives depicting Jesus as healer and wonderworker and to images emphasizing his divinity and transcendence, curiously missing from the new images are representations of Jesus' suffering and death on the cross. Apart from a few extant examples, Christ is generally represented as victorious over death but not undergoing death. The cross dominated both Byzantine and medieval iconography, but early Christian art apparently deliberately avoided any graphic portrayals of Christ's suffering and death. The absence of early images of Christ on the cross continues to amaze theologians and church historians, since the death of Christ on the cross is such a central event in the mystery of Christian salvation. Apart from two extant carved gems dating from the fourth century and a disputed second-century wall inscription in Rome, the earliest images of Jesus crucified date from the early fifth century and are very rare until the seventh century. [...] With the edict of Constantine the display of the cross as a symbol of the crucifixion of Jesus was no longer subject to the derision of Christians living in a pagan world. [...] The Christian iconography of the mid- to late fourth century was considerably enlarged by the development of images related to the gospel accounts of Christ's arrest and trial. Appearing above all on sarcophagus reliefs, these biblical narratives included images of Jesus' arrest, his crowning with a wreath, usually of laurel rather than thorns, Simon carrying the cross, and Pilate washing his hands. None of these scenes, however, included the actual crucifixion. [...] In the fourth and fifth centuries the simple cross appeared frequently".
The earliest images of Jesus crucified date from the early fifth century and are very rare until the seventh century. Only in mediaeval times did artists begin to show Christ on the cross as suffering. But the cross itself, without Christ attached to it, was already displayed in the third and fourth centuries. And all representations were of a cross, not a stake, until the nineteenth and twentieth centuries. The centuries after Constantine's involvement with Church matters Christians included gradually more and more in their worship the sign of the cross.
"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).
"This Pagan symbol [the Christian cross] seems first to have crept into the Christian Church in Egypt, and generally into Africa. A statement of Tertullian, about the middle of the third century, shows how much, by that time, the Church of Carthage was infected with the old leaven. Egypt especially, which was never thoroughly evangelised, appears to have taken the lead in bringing in this Pagan symbol. The first form of that which is called the Christian Cross, found on Christian monuments there, is the unequivocal Pagan Tau, or Egyptian 'Sign of life'. Let the reader peruse the following statement of Sir G. Wilkinson: 'A still more curious fact may be mentioned respecting this hieroglyphical character [the Tau], that the early Christians of Egypt adopted it in lieu of the cross, which was afterwards substituted for it, prefixing it to inscriptions in the same manner as the cross in later times. For, though Dr. Young had some scruples in believing the statement of Sir A. Edmonstone, that it holds that position in the sepulchres of the great Oasis, I can attest that such is the case, and that numerous inscriptions, headed by the Tau, are preserved to the present day on early Christian monuments.' The drift of this statement is evidently this, that in Egypt the earliest form of that which has since been called the cross, was no other than the 'Crux Ansata', or 'Sign of life', borne by Osiris and all the Egyptian gods; that the ansa or 'handle' was afterwards dispensed with, and that it became the simple Tau, or ordinary cross, as it appears at this day, and that the design of its first employment on the sepulchres, therefore, could have no reference to the crucifixion of the Nazarene, but was simply the result of the attachment to old and long cherished Pagan symbols, which is always strong in those who, with the adoption of the Christian name and profession, are still, to a large extent, Pagan in heart and feeling. This, and this only, is the origin of the worship of the 'cross'. This, no doubt, will appear all very strange and very incredible to those who have read Church history, as most have done to a large extent, even amongst Protestants, through Romish spectacles; and especially to those who call to mind the famous story told of the miraculous appearance of the cross to Constantine on the day before the decisive victory at the Milvian bridge, that decided the fortunes of avowed Paganism and nominal Christianity. That story, as commonly told, if true, would certainly give a Divine sanction to the reverence for the cross. But that story, when sifted to the bottom, according to the common version of it, will be found to be based on a delusion..
"When the persecution ceased, and "the Church rose to the kingdom of this world," and the bishops sat enthroned, and dispensed justice and judgement as civil magistrates, and all the power was in the hands of Christian emperors [...] the fabricated wood of the cross multiplied with wonder-working energy in all the world; and the Church, departing from the faith, became changed from the love and worship of God, and the hope of His coming and kingdom, even to reverence and worship the image of the cross, and to enjoy the kingdom of this world, and extend it, with the aid of the dumb idols of the saints and relics of the martyrs." (Henry Dana Ward, The History of the Cross, The Book Tree publ., 1871/1999, p. 66)
"After the conversion, so-called, of Constantine the Great (B.C. 313), the cross first came into use as an emblem of Christianity.
The cross symbol was also used by many different earlier civilizations, including the idolatrous ancient Egyptian and Chaldean religions. See Christian cross#Pre-Christian cross symbols.
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 (see Apocalypse of Mary, viii., in James, "Texts and Studies," iii. 118).
"The early years of the fifth century are of the highest importance in this development, because it was then that the undisguised cross first appears. As we have seen, such was the diffidence induced, and the habit of caution enforced, by three centuries of persecution, that the faithful had hesitated all that time to display the sign of Redemption openly and publicly.
"But the cross was, as we have said, a distinctive symbol of Paganism; [...] In Romanism [Roman Catholic Church], which has retained, or readopted, the forms and principles of the old Paganism, there is the same tendency to make the cross the symbol of spiritual life, and to substitute the natural for the spiritual. It is the recognised symbol of the power and authority of the priesthood of that religion, as it was before of the priesthood of Paganism, and the one, like the other, has sought, and claimed, and, for a time obtained, the dominion of the civilised world. [...] For it was the policy of the teachers of the fourth, fifth and following centuries, in order to make Christianity palatable to the Pagans, to retain as far as possible the Pagan rites, ceremonies and symbols, and simply give them a Christian meaning, as in the case of Gregory's well-known instructions to his missioner Augustine, whom he sent to the Pagan Anglo-Saxons, telling him to allow the latter to retain their ancient rites and customs, but that henceforth they were to do them in honour of Christ and the saints; which was, in effect, to retain the old Paganism and merely call it Christian.
"By the early third century, the cross was the recognized sign of Christianity throughout the Roman Empire and, over the next several centuries, the use of this symbol became so widespread that it is found on most remnants of the era. [Ref3: Marucchi, Orazio, Cross and Crucifix, Archaeology of the Cross, The Catholic Encyclopedia, Volume IV, Robert Appleton Company (New York, 1908), p. 520; p. 526.] Perhaps repulsed by the ignominious nature of Christ’s death, [Ref4: The earliest Christians depicted the Crucifixion only in veiled forms, such as a lamb lying at the foot of an anchor, a dolphin entwined around a trident, and Ulysses tied to a mast. Marucchi, p. 527.] the earliest Christians did not portray his crucifixion. “The custom of displaying the Redeemer on the Cross began with the close of the sixth century”[Ref5: Marucchi, p. 527.] and the first datable manuscript image of the Crucifixion is that found in a Syrian Gospel Book written in 586.[Ref6: Codex Syriacus, 56, preserved in the Laurentian Library at Florence. Marucchi, p. 527; Schacher, p. 487.]
Already by the 14th century, Christian groups like Albigensians were rejecting the adoration or veneration of the cross.
The representation of Christ on the cross has been an important subject of Western art since the early Middle Ages. Concerned primarily with simple symbolic affirmations of salvation and eternal life, and repelled by the ignominy of the punishment, the early Christians did not represent the Crucifixion realistically before the 5th century; instead, the event was symbolized first by a lamb and, after the official recognition of Christianity by the Roman state in the early 4th century, by a jewelled cross. By the 6th century, however, representations of the Crucifixion became numerous as a result of current church efforts to combat a heresy that Christ's nature was not dual—human and divine—but simply divine and therefore invulnerable. These early Crucifixions were nevertheless triumphant images, showing Christ alive, with open eyes and no trace of suffering, victorious over death. In the 9th century, Byzantine art began to show a dead Christ, with closed eyes, reflecting current concern with the mystery of his death and the nature of the incarnation. This version was adopted in the West in the 13th century with an ever increasing emphasis on his suffering, in accordance with the mysticism of the period.
The earliest artistic representations of the Crucifixion show Christ on a cross. If they also show the two thieves, these too are on crosses. Not until the second millennium are the two thieves shown as executed on stakes or trees. Not until the twentieth century is Jesus himself shown (in a Jehovah's Witnesses publication) as dying on a stake.