Father Christiansen had been battling recent health issues, but no cause of death was released. The year 2022 marked his 50th year as a priest, and his 60th year as a Jesuit.

Never one to stray too far from education, Father Christiansen's position at the time of his death was as distinguished professor of ethics and human development in Georgetown's School of Foreign Service and senior fellow at Georgetown's Berkley Center for Religion, Peace and World Affairs.

In that role, his last public comments and writings were about the Russian invasion of Ukraine.

"The international order has been shifting for some time. The Russian-Ukraine War is only the latest disruption," Father Christiansen said during a roundtable sponsored by the National Catholic Register and posted online March 16. "Climate change, the coronavirus-19 pandemic and the rise of China had already begun to re-shape international relations."

"The war in Ukraine," he continued, "sets back the efforts at reducing climate-warming gases, both directly through the pollution created by bombardment, and indirectly through the doubts it places on nuclear power and the scramble it has created over fossil fuels, particularly dirty fuels like those from the Canadian tar sands."
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He added: "The surprising developments emerging from the (Ukraine) crisis, beyond the revival of NATO, include renewed commitment by member states to the U.N. Charter and the values undergirding it, and new confidence, if not in the liberal international order, then at least in the rule of (international) law," he added.

In a March 8 essay for America magazine, he wrote: "The editors of America have asked me whether the Ukraine war is one where the international doctrine known as the 'responsibility to protect, or R2P, might apply as a remedy for this savagery."

"Yes, it does," Father Christiansen said, "because Ukraine needs assistance in protecting its citizens, but it probably does not apply in the direct sense of armed military intervention to rescue those threatened civilians."

Father Christiansen served at America, a national weekly Jesuit publication based in New York, for 10 years, from 2002 to 2012. For the last seven of those years, he was editor-in-chief.

The concepts of just war as they apply to Russia's invasion of Ukraine were mirrored more than 30 years before during a panel discussion on U.S. conduct against Iraq during the Persian Gulf War, that the priest took part in.

During the 1991 parley, the Jesuit priest – then professor at Jesuit-run Santa Clara University – said he was concerned about the U.S. government's apparent eagerness to resort to "mid-level intensity conflict" at any trouble spot in the world with the current lack of a Soviet superpower counterbalance.

He said he feared Catholic just-war teaching "will fail in its attempt to stop war or curb its violence."