In September 1913, Father Eustace went to the Novitiate of the Congregation of the Sacred Hearts in Tremelo, Belgium. He began his novitiate under the supervision of Father Medardo Kaiser and took the name Frater Eustace.

Father Gil van den Boorgart, a fellow missionary of Father Eustace, once said: “"The novitiate that followed his time at our Apostolic School was in Tremelo, in the same house where the aforementioned Father Damien was born, which was converted into a novitiate; this house is in Belgian territory. In 1914, Belgium was invaded by the Germans, and the novitiate in that city ceased; young Hubert had to interrupt his novitiate by going home, but he didn't like this, preferring to be in the neighboring rectory, with its ecclesiastical environment. This situation didn't last long, as he had to resume his novitiate, which he did and completed in the same house where he had studied, since the Apostolic School he had attended had been transferred from there to another location."“", concluded Father Gil.

In January 1915, Eustáquio professed his temporary vows and, in September of the following year, began his studies in Theology. Two years later, in 1918, he professed his perpetual vows.

In May 1919 he received the Order of Diaconate and three months later, priestly ordination, in Ginneken.

“I was present at the priestly ordination and I was the one who gave the occasional sermon at the first Mass. I am therefore qualified to say that in the general joy of those occasions, nothing stood out as exceptional. I noticed that his parents were very moved and wept with joy.” (Father Gil)