A sign appeared to the Roman Emperor Constantine as a light in the noon-day sky. At the same time, he heard the words, In Hoc Signo Vinces (In this sign you shall conquer). He took this as a sign from God and decided the shape drawn by the light should be an emblem for his military. This emblem became known as Labarum (etymology unclear - possibly a Gaulish/Celtic word. See Lauburu Cross).
The shape he saw was of a flaming cross - "a long spear, overlaid with gold". It had a cross bar and "on the top of the whole was fixed a wreath of gold and precious stones, and within this the symbol of the Saviour's name, two letters indicating the name of Christ by means of the initial letters, the letter 'X' intersection 'P' at the centre." (recorded in the 3rd century by Eusebius Pamphili, Bishop of Cesarea in Palestine)
Because this cross is formed with a 'P' and 'X', it is sometimes mis-named as Pax Cross.
Look for 'XP' and 'cross' through an internet search engine, and you'll most likely be directed to pages about Microsoft's Cross-Platform operating system, Windows XP. (Was that an attempt by Microsoft to spread Christ's name throughout the world?) Alternatively, you might find some interesting articles on another XP, Xeroderma Pigmentosum, which is a rare and incurable genetic disorder where people who have it must stay out of the sun. For them, Windows can result in cancer!
The Greek word for Christ is Christos and the Greek lettering is Χριστος. The first letter 'Χ' (chi, shaped like the St. Andrew's Cross) and the second letter 'ρ' (rho), form the monogram of Christ. Add to this Alpha (Α) and Omega (Ω) and we have a complete acronym for the words of Jesus: "I am the Alpha and Omega". (See Alpha & Omega Cross.)
(This is often refered to as a Chrismon. See also Iota-Chi Cross and other Chrismons.) This XP combination also became the monogram of Constantine. Coincidentally, the labarum was similar to an existing pagan emblem used as a standard by the Roman cavalry. Constantine was the Pontifex Maximus, chief priest of the classical Roman Pagan religion. So it's easy to see why he warmed to the symbol.
However, the timing of witnessing the sign was significant; it was 27th October 312, the day before an important battle on Milvian Bridge in Rome, which he won. Since chi and rho form the monogram of Christ, he understood it was a sign from the Christian God and thenceforth Constantine became more tolerant of Christianity. (Some books indicate that Constantine made Christianity the state religion, but that didn't happen until a century later during the reign of Theodosius.) Nevertheless, Jews were required to convert. If they refused, they were regarded as outcasts and Devil worshippers.
During the Second World War, Hitler hid behind the Catholic Church and the Swastika Cross to persecute the Jews. And today, the current bellicose 'war on terror' (the absurd and tragic war on reality) hosted by 'Christian' nations hiding behind their cross, has led to increased persecution of Muslims. There has been slaughter of Muslims by Christians in Kosovo; genocide by Muslims, Roman Catholics, and Serbian Orthodox followers against each other in Bosnia-Herzegovina; mass murder of Roman Catholics by Muslims in East Timor, and...
Like Constantine, all of these people started on a path they had convinced themselves was right. Persecution might not have been on their wish-list, but they had a 'natural' urge as leaders to expand their empire and power. Perhaps 'natural' is an odd choice of words but it does seem, time and again, that many of the elite succumb to greed and wage a war against those beneath. Such people have bowed, perhaps unwittingly, to the Devil. (And for those who don't believe in God and/or the Devil, let's just agree that these people are satisfying their own greed.)
A spin-off of this greed and hunger for power is not only persecution and oppression, but also a tarnishing of certain religions. In fact true followers of these religions are not only tolerant of others, but go further than that. They love other people, even if those other people have different beliefs. (See Interfaith Cross)
Jesus said "Love the Lord thy God with all your heart, all your soul, and with all your mind. This is the first and great commandment. And the second is similar: Love your neighbour as yourself." (Matt. 22:37-39). What an awesome responsibility for us all, especially our world leaders. But what an awesome licence, privilege and opportunity for us all, to show the love of God.
The Chi Rho Cross is a warrior's cross. A cross, like any other we could choose, to follow Christ's example to wage war on any form of evil. And the surest thing to overcome evil, the only thing to overcome evil, is of course, love.