VATICAN CITY (CNS) — To promote Christian unity, Catholics and Orthodox must hold firm to the faith handed down by the apostles and witness together to the Gospel as the good news humanity is seeking, Pope Benedict XVI told Orthodox Ecumenical Patriarch Bartholomew of Constantinople.

“By generously offering their lives in sacrifice for the Lord and for their brethren, the apostles proved the credibility of the good news that they proclaimed to the ends of the known world,” the Pope said in a written message delivered Nov. 30 to the patriarch in Istanbul.

The Pope’s message was carried to Patriarch Bartholomew by Cardinal Kurt Koch, president of the Pontifical Council for Promoting Christian Unity, who led a Vatican delegation to a celebration of the feast of St. Andrew the Apostle, the patriarchate’s patron saint.

The Pope said marking an apostle’s feast day should be a “strong summons” to all Christians “to renew their fidelity to apostolic teaching,” a topic that has become increasingly important in ecumenical dialogue as disagreements increase over what are essential points of faith and what modern adaptations are permissible or even necessary.

In the modern world, the Pope wrote, it is especially important for Christians to work more closely in sharing the Gospel and in presenting Christ “as the answer to the deepest questions and spiritual aspirations of the men and women of our day.”

While Cardinal Koch joined the ecumenical patriarch’s celebration in Istanbul, Cardinal Tarcisio Bertone, Vatican secretary of state, celebrated the feast with Catholics and Orthodox in the Orthodox cathedral of Astana, Kazakhstan.

Cardinal Bertone gave the homily and then presented Kazakhstan’s Christian leaders — Catholic Archbishop Tomasz Peta and Orthodox Metropolitan Alexander — with fragments of the relics of St. Andrew. Cardinal Bertone was in Kazakhstan to represent the Vatican at a meeting of the Organization for Security and Cooperation in Europe.

The cardinal said the Church leaders had asked Pope Benedict for a part of the relics, which are housed in Amalfi, Italy, to “underline our common veneration of the apostles.”