At the same university Callus undertook studies in philosophy, history of art, and Semitic languages.
Here, he gave lectures and prepared the examination ad gradum for the Master of Sacred Theology degree which he obtained in 1924 at Rome’s Pontificium Collegium Internationale Angelicum, the future Pontifical University of Saint Thomas Aquinas, Angelicum, (the degree was officially conferred a year later).
On May 1, 1924, Callus was honored with the title "Master of Sacred Theology" bestowed by Procurator General of the Dominican Order.
In 1931 Callus resigned from all of his offices in Malta and permanently left for Hawkesyard, and then, a year later, for Oxford, England, where he stayed until his death.
Callus thus continued to mature his philosophical views and to extend his already wide horizon of knowledge.
Callus traveled far and wide lecturing and addressing seminars and conferences on Medieval philosophers, and expounding his own philosophy.
He was buried at the cemetery of Żebbuġ, Malta, his birthplace, following an impressive funeral at the church of St. Dominic’s in Valletta.
These works are generally distinguished under three main headings, as indicated hereunder: These are contributions on themes related to Medieval texts and manuscripts linked to Holy Scripture or Biblical studies.