In the Basilica built in 1913 by Pope Pius X in the vicinity of Ponte Milvio to commemorate the sixteenth centenary of of the Edict of Milan (313) in the place where, according to tradition, Emperor Constantine I sounded the trumpets to announce to the city the end of hostilities against Christians, Fabro lived and exercised his priestly ministry for more than 50 years. Still cleric, he moves to Santa Croce in 1934 to be the organist of the church; the pastor at this time is Father Emilio Recchia. Ten years later he returns as Superior of the Community, a position which is lifted after three years so that he may devote himself to intellectual activity and teaching. Nonetheless, Fabro preserves throughout his life the valuable appointment of the Sunday noon Mass as well as confessions and some activities in the oratory.

He remains in Rome even during his last illness, after having expressed his desire to die in Rome, the land washed by the blood of so many martyrs; and his body is buried at the Verano Cemetery.