When Father Eustace, then 14 years old, chose the Congregation of the Sacred Hearts, opting to live a missionary life, serving God and neighbor. In 1915, the young Dutchman professed the vows of Poverty, Chastity, and Obedience, becoming a religious of the Sacred Hearts, and in 1919, he received priestly ordination.

During his early years in the Congregation, Father Eustace served God's people in the Netherlands on various missions, distinguishing himself through his work. For his life dedicated to serving others, he received a decoration from the King of Belgium for services rendered to Belgian refugees during the First World War.

But it was in 1925 that Father Eustáquio received a great challenge, one that would mark his missionary life. He, along with Father Gil van den Boogaart and Father Matias van Rooy, were the founders of the Congregation of the Sacred Hearts in Brazil, in the Triângulo Mineiro region. From there, Father Eustáquio served in various Brazilian cities, making a difference in the communities and in people's lives.

His missionary work was one of the subjects the Dutch priest discussed with his family in the letters they exchanged. In 1936, Father Eustace wrote to his sister, Faustina, also a nun, about the joy of living a life dedicated to others.

“When I think of Divine Providence, my heart is filled. I feel, then, more than ever, that a solicitous Father takes pity on the weary missionary to console and strengthen him. How beautiful is the missionary life, yet how little it is all worth without divine consolation and help. I will never tire of praising and singing of Divine goodness.”