He was one of the many Jesuit missionaries who strove throughout Germany, from Freiburg to Berlin and Danzig, to reawaken and strengthen the country's Catholic forces after the stormy year of 1846.
During these years he underwent a spiritual change, and in particular, by studying the Church Fathers, stirred his mind with theological knowledge; after his liberation he entered, in the spring of 1840, the novitiate of the Society of Jesus, at Saint-Acheul, France.
It was at this time that the popular missions were inaugurated in Germany, but Hasslacher's delicate health could not long withstand the physical exertions entailed.
Instead, as he explains in an 1860 letter, Hasslacher began giving conferences in all the larger cities in the Rhine and Westphalia.
His strength failing, he was sent in 1863 to conduct, in Paris, the St. Joseph's Mission for German Catholics, but even this labour became after ten years too much of a tax on his physical powers so that he was compelled to abandon it and to take up similar but lighter duties at Poitiers.