Subsequently, he followed the usual cycle of studies at the major seminary itself and in 1990 gained the pontifical baccalaureate in theology from the Facoltà Teologica dell'Italia Settentrionale.
On June 8, 1991, he was ordained priest at Milan for the diocese by the then archbishop, Cardinal Carlo Maria Martini, a noted biblical exegete.
Under the direction of Fr Albert Vanhoye, S.J., later made cardinal by Pope Benedict XVI, he prepared the required thesis for the licentiate, entitled 'L'opera salvifica di Gesù Cristo tra kenosi (Fil 2,6-11) e solidarietà (Eb 5,5-10)' (The Salvific Work of Jesus Christ between Kenosis (Phil 2:6-11) and Solidarity (Heb 5:5-10)).
Confronto esegetico-teologico di Fil 2,7 con Lc 1,48' ('Obedience as Form of the Service of Jesus Christ and of Mary: Exegetical and Theological Comparison between Phil 2:7 and Lk 1:48'), of which an extract was published in July that year.
In his later research and publications, Manzi has generally not pursued further investigation of the world of Qumran, which is not lacking in the attention of other scholars, but has been occupied with various themes connected with the Letter to the Hebrews and angelology, as well as the Pauline corpus.