Athenagoras was born as Aristocles Matthew Spyrou on 6 April 1886 (25 March) in the village of Vasiliko, near Ioannina, Epirus (then Ottoman Empire).

[2] After completing his secondary education in 1906, he entered the Holy Trinity Theological School at Halki, near Constantinople, and was ordained a deacon in 1910.

[citation needed] Archbishop Athenagoras consecrated the Archdiocesan Cathedral of the Holy Trinity on New York City's Upper East Side on 22 October 1933.

[9] In January 1949, he was honored to be flown in the personal airplane of the American president Harry Truman to Istanbul (Constantinople) to assume his new position.

It produced the Catholic–Orthodox Joint Declaration of 1965, which was read out on 7 December 1965, simultaneously at a public meeting of the Second Vatican Council in Rome and at a special ceremony in Istanbul (Constantinople).

There was one who strongly objected: Metropolitan Philaret Voznesensky of the Russian Orthodox Church Outside of Russia challenged the Patriarch's efforts at rapprochement in an open letter in 1965.