Fabro attends the School of Philosophy at Saint Apollinaris College (today the Pontifical Lateran University). With only two years of University, he obtains his doctorate at the Pontifical Lateran University, where he will continue his successive academic career.

At Genzano (Rome), he pronounces his perpetual profession on August 25, 1932 (delayed due to his age, the Chronicles note), which is followed in Rome by the ministry of Lector (November 3, 1933), and of Acolyte on the same day, and that of Deacon on November 11, 1934.

On the 20th of April, 1935, Holy Saturday, he was ordained a priest in the Basilica of St. John Lateran, upon a prior dispensation of five months from the canonical age.

In 1935, he obtains the title of Licentiate in Theology at the Pontifical University of the Angelicum. He obtains the Doctorate in Theology at the Pontifical University of the Angelicum, with the grade Summa cum laude in 1937, with the theological dissertation: “La nozione metafisica di partecipazione secondo San Tommaso D’Aquino—Saggio di introduzione analitica al pensiero tomista”.

He begins his teaching career at the Pontifical Urbaniana and Lateran Universities, the Libera Università Maria Santissima Assunta (LUMSA) at Rome, and the Sapienza University of Rome.

At Rome he collaborate as a consultant at the Second Vatican Council and with the Congregation for the Doctrine of the Faith and with the Congregation for the Causes of Saints. He lived in Rome until his death.