Α theologian and professor. He studied at the Theological School of Halki (Turkish:Heybeliada) in Istanbul and then continued at the Oxford University and Humboldt University of Berlin.  He was a professor of Theology at the University of Thessaloniky in the years 1942-1951 and later at the Kapodistrian University of Athens from 1952 till 1963. His research was focused on the analysis and the understanding ( hermeneutics) of the New Testament. Ioannidis also participated in first three Assemblies of the World Council of Curches (1947, 1955 and 1961) and was involved in the movement of Ecumenism.

He cooperated with the reformist in the field of education and supporter of the Demotic Greek language, Alexandros Delmouzos. He is considered among the most important 20th century theologians in Greece, along with the former Archbishop of Athens Ieronymos I, Christos Androutsos, Panagiotis Trembelas, Panagiotis Mpratsiotis and Ioannis Karmiris.