BERLIN, Germany — Germany’s two main Christian churches will lose millions of members over the coming decades and will have to tighten their financial belts as a result, according to a study published Thursday.

Researchers at the University of Freiburg predict that the combined membership of the German Catholic and Protestant churches will drop from about 45 million now to 34.8 million by 2035 and 22.7 million by 2060.

The study, which was commissioned by the Catholic German Bishops’ Conference and the Evangelical Church in Germany, blamed the decline on a number of factors, including more members dying than children being baptized, and adults deciding to leave the church.

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Members of the Catholic and Protestant churches in Germany pay up to 9 percent of their taxable income to the church, generating billions of euros a year for the two institutions.

Low unemployment and steady growth in Europe’s biggest economy have boosted the two churches’ combined income to around $13.5 billion in recent years. But the study predicts that inflation will push churches’ spending on salaries and building maintenance to nearly $28 billion by 2060, creating a major shortfall.

“These changes will happen and it’s good to focus on the questions of tomorrow during the present economically good situation,” said Bishop Heinrich Bedford-Strohm, who heads the Evangelical Church representing various strands of Lutheran and Reformed Protestantism.

“We won’t panic over the projection but align our work accordingly,” said Cardinal Reinhard Marx, chairman of the German Bishops’ Conference, adding that he sees the study as “a call to missionize.”

Recent immigrants to Germany, who have swelled the population to about 82 million, have largely been Muslim or haven’t registered as church members, though the Catholic Church has gained some members from Eastern European migrants.