One of the great gifts for a busy parish priest -- and these days most parish priests are very busy indeed -- is the gift of a retired or elderly priest who offers to "cover" the parish for a few weeks so that the pastor has time for some rest.

Looking at the pictures of Fr. Jacques Hamel, the elderly French priest brutally slaughtered by ISIS on Tuesday while celebrating Mass in church, I couldn't help but think of what the pastor of the parish must be feeling.

Fr. Jacques was one of those generous, gentle, dedicated men for whom the word "retirement" was a foreign concept -- you can see in the photographs his gentleness and humility. He was "supplying," as it's known in the church, for the pastor, to ensure that people were able to receive the Eucharist. He was displaying the very essence of the priesthood.

Fr. Jacques was martyred -- and please, let no one speak of "workplace violence," mental illness or poor social conditions. He was martyred for one reason: because he was a Christian and a priest. Fr. Jacques was martyred in a sacred space dedicated to the worship of Almighty God, what the church calls the "Domus Dei, the House of God."

His throat was cut while he celebrated Holy Mass which, for a believing Catholic or members of the Orthodox Church, is not a re-enactment of the Last Supper and the Sacrifice of Calvary. But it is a sacred timeless moment, God made present in time.

He was not the first priest to be martyred at this profoundly holy moment -- and he will most surely not be the last.

The city in which I was educated and formed as a priest, Canterbury in England, boasts one of the greatest saints in the life of the church, St. Thomas Becket, Archbishop of Canterbury. Becket was martyred in his own cathedral at the order of the king.

Archbishop Oscar Romero, of El Salvador, whose relics are enshrined in Canterbury's Catholic Church, was shot within moments of uttering the words of consecration.

Fr. Ragheed Ganni was shot along with three sub-deacons outside his parish in Mosul, Iraq, on Trinity Sunday in 2007, just moments after celebrating Mass. His killers, who were followers of what self-serving politicians call "the religion of peace," had demanded that he close the church.

As they prepared to martyr Fr. Ragheed, they asked him: "Why didn't you close the church?" He responded, "How can I close the house of God?"

Fr. Ragheed was the secretary of Paulos Raho, the Archbishop of Mosul -- and nine months after his secretary's murder, Archbishop Raho was himself murdered.

It is not possible to list the names of all the priests martyred while celebrating Mass, not only because there are too many but because, as the great French writer Francois Mauriac said, this will not end until the last Mass is celebrated on Earth.

I was most grateful that at my last parish in the United States, thanks to the Second Amendment, my parishioners had the right to bear arms legally. I knew that on any given Sunday, several of the faithful were "packing." Because of my work for the persecuted Christians of the Middle East, in fact, one of my newest parishioners told me a few weeks ago that he decided to "carry." I don't think he was referring to the missalette.

Some will be shocked that people carry guns at Mass. But they have obviously never been to Mass where there is the possibility of armed assailants entering the church and killing the priest. Try going to Mass in Baghdad -- or Normandy.

Two months ago, Patriarch Kirill, the head of the Russian Orthodox Church, declared that the war on terrorism and on ISIS specifically a "holy war." It was, he said, a fight against a "fearsome foe, threatening the whole of humanity."

Unlike other Christian leaders, who have -- through negligence, weakness, or willful naivete -- their heads buried firmly in the sand, Patriarch Kirill has done a prophet's work: He sees the evil of the day in a clear light.

Fr. Benedict Kiely is a Catholic priest and founder of Nasarean.org, which is helping the persecuted Christians of the Middle East.