Christodoulou was born in 1856 in Agios Theodoros of Imbros, then Ottoman Empire (now Turkey).
[1][2] Then, for 1 year, he taught theology in the Zappeio Girls' school of Constantinople.
[1][2] From 1899-1904, he worked on a committee assigned by the Patriarch of Constantinople to create a Byzantine text-type New Testament, later published as The New Testament, Approved by the Great Church of Christ in 1904.
[1][2][4][5] Since 1909 he became the Metropolitan bishop of Serres and at the same time helping people suffering from Cholera.
[1][2] During the First World War in 1917, when Serres was temporarily occupied by the Central Powers, Metropolitan Apostolos was not permitted to get out of his cathedral after an order by the Bulgarian commander.