I've at time been puzzled at the notion that Christ became sin for us. For the longest time I thought it meant something that, in the sense of the scapegoat, that all the sins of Man were heaped upon Christ and He dumped them into the oblivion of the grace. I think that's true, but that's not what St. Paul seems to be talking about. Elsewhere, the Apostles describes Christ bearing the sins of the World or taking to the cross for the forgiveness of sins. But in this: He became sin.
One idea I had was that this has to do with Christ assuming Human flesh. Among Protestants, there's a heavy emphasis on the cross, even when discussions are given about the Incarnation. This is not completely incorrect. But what if we see that what Christ is doing is not necessarily something unique, but the nature of His person and office is what makes the event unique. Christ going to the cross isn't that important, but Christ going to the cross is the entire world. Let's explore this more.
The cross is a symbol that is not unique to Christianity. The four-points of the cross is a pretty important symbol throughout many astrological societies. It calls upon the number four, which denotes a completeness over all of creation. It also has a sub-divison, two and two, which implies parallel binaries. Then of course, the cross possesses dual axes, one going up-down, and the other side-side (or left-right if you will), overlapping with a center point.
The Early Church had no problem in thinking about the symbolic import of why God Incarnate died on such a death instrument. They didn't see it as a Pagan cross-reference, but that God's Wisdom was even present among those stumbling in the Dark ("He's not far from anyone"). Of course, the more wise of them realized this was a secondary issue beneath the primary symbolism of the Cross, as wood, being the very tree in the Garden (conveyed by the Apostles themselves). But the Fathers believed God was working within all details of Scripture, there were no accidents.
I don't plant to give a large exposition of this, since it's all speculation. But I think an important point to note is that Christ going to the cross was not God taking the punishment we were yet to take, but that God took the punishment we were already suffering. Adam and his race was already cursed since Genesis 3 (ending in death), and this is the curse/punishment that Christ sought to take upon Himself. Thus God redeemed Man by joining us in our curse. This makes more organic sense (and follows Scripture better) than the arbitrary notion that the cross is what we deserve. This is a Medieval invention that sounds awfully like the sadistic torture princes would afflict people with and call it justice.
But, given the above, why the cross? Well, what if Human life is summarized as a cross?
Perhaps, the horizontal axis represents our own temporal displacement between past and future. We are haunted by the past and tormented by the future. We've felt loss and expect the looming shadow of death. As Mankind hangs on the cross, we are torn between these two poles of pain as we dwell in the present. The nails represent the unchangeability in either direction. Our past is fixed and our future destined. We can't go back and fix our mistakes and we can't stop the oncoming of death.
Perhaps, the vertical axis represents our ontological or topological (spacial) displacement between Heaven and Earth. I'm talking about Heaven as a spiritual abode for spiritual beings (angels and demons). As Man we are tapped into two realms, and our disconnection leads us towards many perversions. On the one-hand, we lunge downwards and become like animals. On the other hand, we try to pull upwards and become demonic. Of course, perhaps its fitting that the feet are nailed, fixing us in our animality (we never can cease to eat or sleep). But our gaze upwards never can quite get there, and attempts to do so make demonic ascetics who are cruelly inhuman.
It's this that Christ joins, and in being crowned as King of Cursed Man (prophetically in the crown of thorns), He becomes truly Man for all Humans. It's this very fact that saves Humankind. It's who joins us in our place and takes up our cause. Christ has taken up our curse, in fact He has become our curse, cementing it into a horrifying and obscene act: crucifixion. It's an enacted allegory for the Human condition, described symbolically above.
So it might be strange to think about, but all of Human life is a kind of crucifixion. But instead of merely suffering our curse, there is redemption. We can take up our crosses and follow Christ instead. The suffering of Human life, the common pains of life, can be transformed and transcended. This is the great blessing of Christ. He became Sin, and now we might become righteousness.