Advocates for persecuted Christians said Walther's role in the region was estimable.
Father Benedict Kiely, founder of an organization dedicated to assisting persecuted Christians in the Middle East, recalled that Walther made a concrete difference in difficult and complex situations.
"His influence in Iraq was astonishing," said Father Kiely, a priest of the personal ordinariate of Our Lady of Walsingham. "He truly had a passion for the persecuted and used all his considerable skills to raise the issue and provide support."
Walther played a central role in moving U.S. policy in favor of protecting Christians in the Middle East, said Nina Shea, an expert on religious freedom at the Hudson Institute.
Walther organized a media awareness campaign, and directed a critical fact-finding report to prove that Christians had been targeted by ISIS for genocide in Iraq and Syria.
Kathryn Jean Lopez, director of the Center for Religion, Culture, and Civil Society at the National Review Institute, praised Walther's wisdom and prudence in working with both the Church and the U.S. government for the common good.
Praising Walther's "knowledge and experience and connections," Lopez added that "honestly no one but God could ever know the extent of his interventions."
"He was a problem solver, he was always seeing things many steps ahead. He had a wisdom about him that always seemed to me of the Holy Spirit."
Robert Nicholson is executive director of the Philos Project, which advocates on behalf of Christians in the Near East. Nicholson said that despite Walther's accomplishments in helping preserve Christianity in the region, he never sought the spotlight.
"Some leaders like to shout their achievements from the housetops, but that wasn't Andrew," Nicholson told CNA in a statement. "He was the guy working behind the scenes to build coalitions, catalyze other leaders, and find practical means for pursuing lofty ends."
Catholic leaders agreed that Walther's role was critical in helping preserve a Christian presence in the Middle East.
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"There is nobody who worked harder and with more hope and perseverance on our behalf," Archbishop Warda said.
"We may never know how many Christians are living safer, better lives in the Near East today because of this one man," Nicholson said.
Lopez noted that "if Christianity survives in Iraq and Syria -- the cradle of Christianity -- it will be in no small part because of his efforts."
The National Catholic Register contributed to this story.