INRI: The Mysteries and Meanings of the Cross of Christ
In the Christian scheme, there is a break, a bright hard line in history, represented by the direct intervention, the gratuitous (freely, graciously given) personal revelation of the Creator in the lives of real human beings.
This event is reckoned as the most important of all events: the incarnation of the only begotten Son of the only Father God as a man, Jesus bar Joseph of Nazareth, who came to live and die as one of us. You and me. It is a continuing divine revelation in the sense that an assembly of believers has remained in existence ever since, continuously, claiming the protection and guidance of the Holy Spirit, and passed on to succeeding generations the doctrine (that is, the truth) of that greatest-ever happening.
Believers have recorded and codified the manifestation of the Event in the four Gospels of the canon (the small library of books generally accepted by the church's ministerial leadership or hierarchy) that dates to within two centuries of the life, ministry, death and resurrection of the same Jesus. The crux of the case for Christianity is the death of the God-turned-man--and his triumph over that death.
Thus, the symbol and source of the Christian belief is the crucifixion, the image of a man writhing in utter agony on a cross of wood, which represents the Roman (read: "human") genius for torture and execution.
Essential, as well, to the understanding of core Christian belief, especially as represented in the Gospel writings, is the concept of mystery.
The Catholic and Orthodox churches, which lay claim to being the oldest in direct apostolic succession among all the Christian churches and denominations, place great emphasis on the "mystery of faith" and the "mysteries" of the life, death and resurrection of Christ. In this context mystery means "not fully knowing" and "not capable of being fully known" by human reason. It does not mean "cannot be known" or "not ever to be fully understood or revealed." Which returns us to the equally poignant concept of time or "salvation history."
Thus the "mystery of the cross" is an invitation for believers to reflect on the willingness of their Creator to suffer the worst degradation imaginable by someone like them--and so unlike them so as to redeem them from their sins.
Christian traditions other than Catholic and Orthodox, such as Bible-based Protestant churches and modern-day evangelicals, may choose to emphasize aspects of the crucifixion other than mystery, per se, though all who believe that Jesus is the long-promised Messiah acknowledge the leap of faith - over the chasm of the unknown and unbelief necessary to achieve the personal relationship with the Savior that fulfills the resurrection-promise of the New Testament. In fact, in the Catholic and Orthodox traditions, depictions of Jesus' death dominate depictions of his resurrection.
The death on the cross is the one moment, memorialized on Good Friday in the Church calendar, of the utter humanity, complete and irrevocable, of the Messiah. That point is a pivot from which all else follows in the Christian world view.
What creates this bright-line demarcation on the continuum of history? The four Gospels attributed, in chronological order of their writing, to Mark, Matthew, Luke and John, tell the story as it was handed down (perhaps witnessed by one of the evangelists, John). The symbolism emerged in the early centuries of the life of the Christian Church, though it wasn't always the dominant image: Think of the Chi-Rho or the ubiquitous fish.
But through the ages artists seized on this pithy and violent depiction of the one moment and found a bottomless appetite for the image. Every Catholic Church in the world displays the crucifix, as does every Rosary.
There is hope that we can know, but it requires of the believer a total investment of body and soul - and it requires of God, through the inspiration of the Holy Spirit, to move actively in that same body and soul. At least, that is a Christian-Trinitarian formulation of the process of an individual's conversion to belief, to a way of death, which leads to eternal life, represented by the image of the man on the cross, flanked by two thieves or revolutionaries and labeled with a mocking sign.
INRI: Iesus Nazarenus Rex Iudaeorum: Jesus of Nazareth, King of the Jews. He died under Pontius Pilate, then . . . well, you know the rest of the story.