Aboard the papal flight from Romania — Pope Francis has criticized traditionalist Catholics who seek to "safeguard the ashes" of the past, saying the global church's centuries of tradition are not akin to a container for preserved objects but instead like roots to be drawn on for future growth.

"I feel the nutrition coming up through the roots, and it helps me go forward," said Francis.

Francis repeated his frequent characterization that having Benedict live in retirement at the Vatican is for him like having a grandfather at home. Francis said that going to speak with his predecessor always gives him strength.

Francis spoke about the church's tradition after being asked about his relationship with retired Pope Benedict XVI.

Francis did not specify in his remarks whom he had in mind while speaking of Catholic traditionalists. But the pontiff has come under criticism during his six-year papacy from a vocal minority of Catholics, including some cardinals , who have expressed concern that he has not been clear in enunciating some of the church's teachings.

"The tradition of the church is always in movement," he said. "The tradition does not safeguard the ashes."

"Tradition is like roots [of a tree], which give us nutrition to grow," he explained. "You will not become like the roots. You will flower, grow, give fruit. And the seeds become roots for other people."

"Tradition is the guarantee of the future and not the container of the ashes," said Francis.

In a press conference aboard the papal flight back to Rome after his three-day visit to Romania, he singled out Catholic "fundamentalists," who he said have a nostalgia for "returning to the ashes."

Among other issues he spoke about in the half-hour press conference, the pope also made an impassioned plea for Catholics to pray for the future of the European Union, which he said has "aged" and is in danger of being "overcome by pessimism."

In his first remarks about the 28-member block since European parliamentary elections last week resulted in gains for populist parties across the continent, the pope lamented the appearance of what he called "new borders" across the EU and said the organization "has lost the goal of working together."

"Europe is not being attacked by cannons or bombs in this moment but by ideologies, ideologies that are not European, that come either from outside or that stem from small groups," he said.

After referring to the tragedy of the two World Wars, he pleaded: "Please, let's not return to this. Let's learn from history. Let's not fall in the same hole."

"I would say to believers, 'Pray for Europe … for unity, that the Lord will give us this grace,' " the pope added later.

Francis had been in Romania May 31-June 2 for a visit that focused both on encouraging the country's minority Catholic population and reaching out to the 16-million-member Romanian Orthodox Church.

Asked about his May 31 meeting with Romanian Orthodox Patriarch Daniel, Francis called him "a man of great heart, and a great scholar."

"He is also a man of prayer," said the pope. "It is easy to get close to Daniel because I felt that he was a brother."

Francis said that ecumenical relations between Catholics and Orthodox are built by walking together, and by jointly serving those in need.

"There is already Christian unity," he said. "Let's not wait for the theologians to come to agreement on the Eucharist."

"The Eucharist is celebrated everyday with prayer, with remembrance of the blood of our martyrs, with the works of charity, and also wanting each other to be well," he added.

Francis also addressed the prayer service he and Daniel hosted together, in which the two leaders both recited the Our Father prayer in each other's presence, but separately — the pope going first in Latin, followed by the patriarch in Romanian.

Some right-wing Orthodox consider it inappropriate for believers to pray with Catholics.

The pope revealed that while the patriarch said the prayer in Romanian, Francis had said it again quietly in Italian. He said that from what he could see, "the majority" of the crowd gathered for the event at Bucharest's new Orthodox Cathedral had prayed both times.

"The people went beyond us leaders," said Francis. "We leaders must have diplomatic balance, to ensure that we are going forward together. There are diplomatic rules that are good to safeguard, so as not to ruin the thing. But the people pray together."

Making his first reference in the press conference to Catholic "fundamentalists," the pontiff said that even some Catholics are "closed" on the issue of praying with Orthodox, thinking them schismatics.

"There are Catholic groups who are a bit fundamentalist," said the pope. "We must tolerate it, praying for them, so that the Lord and the Holy Spirit soften their hearts a bit."

[Joshua J. McElwee is NCR Vatican correspondent. His email address is jmcelwee@ncronline.org. Follow him on Twitter: @joshjmac.]