ISTANBUL, Turkey — Sometimes what a pope doesn’t say can be just as important as what he does, and such was the case in Turkey on Sunday as Pope Francis laid out his vision for unity between Catholics and Orthodox Christianity.

Francis offered several motives for pursuing closer ties, yet conspicuously absent was the imperative most often cited by more conservative Catholics and Orthodox: Making a common stand against secularism, especially permissive sexual morality.

In effect, the pope’s case rested not on the wars of culture, but on the social gospel.

The official reason for the pontiff’s Nov. 28-30 trip to Turkey was to meet the Ecumenical Patriarch of Constantinople, Bartholomew I, who’s considered the “first among equals” of Orthodox leaders.

On Sunday, the pontiff took part in an Orthodox liturgy at the Church of St. George in the Phanar, the headquarters of the Patriarchate of Constantinople, which provided him with a platform to lay out his ecumenical vision.

Francis voiced clear support for “full communion,” meaning that Orthodox and Catholics would recognize a common set of teachings and sacraments and a common governance structure. In essence, they would see one another as members of a single Church.

Knowing that concerns about papal power have long been a stumbling block, Francis insisted that full communion “does not signify the submission of one to the other, or assimilation.”

He then ticked off three reasons why Orthodox and Catholics should come together: to defend the poor, to end war and heal conflicts, and to help young people to see past materialism and to embrace a “true humanism.”

“There are too many women and men who suffer from severe malnutrition, growing unemployment, the rising numbers of unemployed youth, and from increasing social exclusion,” Francis said.

“We cannot remain indifferent before the cries of our brothers and sisters,” he said. “They ask us to fight, in the light of the Gospel, the structural causes of poverty: inequality, the shortage of dignified work and housing, and the denial of their rights as members of society and as workers.”

He was equally passionate about war.

“Taking away the peace of a people, committing every act of violence — or consenting to such acts — especially when directed against the weakest and defenseless, is a profoundly grave sin against God,” he said.

“The cry of the victims of conflict urges us to move with haste along the path of reconciliation and communion between Catholics and Orthodox,” he said.

In a last-minute addition to his text, Francis also recalled the victims of an attack on a mosque in Kano, Nigeria, on Friday that left more than 100 people dead, calling it a “very grave sin against God.”

In a joint declaration that Francis and Bartholomew signed today, they added one more motive for coming together: Defending persecuted believers in the traditional heartland of the faith, saying “we cannot resign ourselves to a Middle East without Christians.”

It’s instructive to compare the pope’s vision with the arguments laid out by a prominent Russian Orthodox leader who was invited to address a recent Synod of Bishops in the Vatican.

On that occasion, Metropolitan Hilarion of Volokolamsk, head of the Moscow Patriarchate’s department for external church relations, argued that Orthodox believers and Catholics must stand should-to-shoulder against the secular tide.

In today’s society, Hilarion said, “there is an increasingly aggressive propagation of the idea of moral relativism applied also to the institution of the family held sacred by all of humanity.”

Among the challenges, he said, are “the disintegration of families, abortion, the legalization of same-sex unions, and the spread of technologies unacceptable from a Christian point of view such as ‘surrogate motherhood’.”

Hilarion called on Orthodox and Catholics “to join efforts and come out as a united front for the noble goal of protecting the family when confronted by the challenges of the secular world.”

The conclusion Francis and Hilarion reached is the same, which is support for unity, but the logic getting them there is clearly different.

For some time, tradition-minded Catholics have generally prioritized outreach to the Orthodox and to Evangelicals, as opposed to Anglicans or mainline Protestants, in part because they find views such as those expressed by Hilarion closer to their own.

Instead, Francis on Sunday laid out a version of the argument for ecumenism that might be called a social gospel — a Christian peace-and-justice agenda.

It’s an approach that’s congenial to Bartholomew, who among other things has been dubbed the “Green Patriarch” for his strong environmental concern. How well it will play in more traditional centers of Orthodox opinion, such as the Russian and Greek Orthodox churches, remains to be seen.

In terms of population, Russia is by far the biggest among the Orthodox churches. Of the roughly 225 million Orthodox believers worldwide, some 150 million, or two-thirds, are Russian Orthodox.

To some extent, the outcome may depend on whether Catholics and Orthodox come to see the contrasting visions presented by Francis and Hilarion as an either/or choice, or a both/and combination. If it’s the former, ecumenism may still face an uphill climb; if the latter, it could attract a broad coalition in favor of rapid progress.