The cross as a symbol of Christianity and as a central image in people's interpretation of the Gospels is a great example of how the meaning of words have developed over time, disconnecting modern Christianity from its roots in Christ's teaching. The "cross" as it is known today was have been completely unknown in Christ time and in the centuries after.
A Stake or Pole
People in Christ’s times not have understood the translation of Matthew 16:24 as we translated it today: “If any [man] will come after me, let him deny himself, and take up his cross, and follow me.” (click to see Greek). They would not have had any context for “taking up a cross”, meaning the instrument of torture, because Christ had not carried his “cross”. None of his listeners knew that he would be hung on a stake, much less be forced to carry it. Christ was saying something completely different than that had nothing to do with the modern "X" symbol.
Instead, his audience would have heard this as the much more understandable: “If anyone wishes to make his way after me, let him reject himself and pull up his stake and be guided by me.” The “stake” referred to the center pole of a tent or the supporting pole of a wall or fence, and the phrase, then as now, was an analogy for being willing to move from where you were to somewhere new. However, Chris also knew that he would die on a pole. So his use of this metaphor was a play on words.
Pull up a Stake or Bearing a Stick
The English phrase, "pulling up stakes" capture many aspects of the Greek meaning. The general idea of both is similar if not identical. The "stake" sets a place of residences and of ownership, whether it supports a building or creates a fence or marks a claim. In a building, it supports a wall. In a fence or a claim, it defines a boundary. In a foundation, it creates a point of support. In all of these examples, it solidifies an existing established place in the world.
When Jesus talks about "lifting up a stake" he destroying that established position. The word translated in Mark 8:34 and Mark 10:21 as "take up", is (ἀράτω) airo, which is often used by Jesus to mean "remove" and is frequently translated that way in the Gospels. Notice how badly the meaning of "remove your cross" works and how well "remove a stake" works. It doesn't make any difference if we are talking about destroying the integrity of the wall, the fence, or the boundary. Lifting or removing a stake destroys the established position, the comfortable place we claim. The use of airo is also problematic with a torture stake since it means both "lifting it" to carry it and "raising it" to plant it in the ground.
In Matthew 10:38, the word translated as "taketh" is lambano (αμβάνει) to "take", "take hold of", "grasp", The imaged here is the "stake" as a "pole" or walking stick. Of course, taking a walking stick to follow someone makes perfect sense alone. It implies that the journey will be long. However, among nomadic people who use simple tents when traveling, the idea of pulling up the central pole and a walking stick are connected. Tents have one central pole that is used as a walking stick when moving, You can see this among the Masai today. This concept flows naturally from the idea of following someone, especially in ancient where so many lived as nomads, shepherds, and traveled the crossroads Again, the idea of "lifting up" the stakes or poles on which the Romans hung criminals would not come to mind at all in thinking of following someone except after Jesus's death, when we know how he died.
Finally, we have the word in Luke 14:27, which is translated as "bear". This is (βαστάζει) bastazo, which means "to lift up", "to raise", "to bear", "to carry", "to endure," and "to carry off, "produce", "yield," of land." Notice that the "bear" here means "produce" and "yield" as well as "carry". However, the primary meaning is "lift up" and the word also means "carry off". Again, none of these ideas are easily applied to the instrument of death. Though Jesus carried the cross, criminals were frequently hung on crosses others carried and planted
The fact that this basic idea of pulling out of an established position works in both English and ancient Greek shows how universal this idea is. It is a deep idea, not one to be taken lightly.
The Meaning of "Cross" as an "X"
The conversion of the word to "cross" started first in the Latin Vulgate, the translation of the Bible into Latin. It translated the Greek stauros into the Latin crux. This word is our phonetic basis for the word "cross" but crux did not mean "cross" either, not as we use it today. It was the Latin word for the form of torture used to kill Christ. It referred to the stake. Its verb form, crucio, means simply "to torture."
So, where did the form of the cross, an upright pole with a crossbar, come from. The early symbols (2nd century) of Christ were a dove, a fish, a ship, a lyre, and an anchor. The Greek letter chi, which looks like the letter X is the first letter of the Greek word from Christ, χριστός. We see early uses of the various Christ symbols with the "X", two fish forming an "X", or the anchor, with its crossbar leaning into an "X.
The symbol of the X grew in popularity with its adoption by Constantine, the first Christian emperor's use of the chi-ro (which look like a P with an X across the descender of the P) on his banner and the painting of the X on his men's shields. This chi-rho image was seen a symbolic representation of the head, arms, and legs of an upright stake. Over time, this image was simplified into the cross we know today.
The earliest image of Christ hanging on a cross was used to decorate a reliquary around AD 420 among The Maskell Ivories of British Museum. See this article tracing the evolution of these images.