Theresa left the Oblates and Baltimore in 1845 and traveled to Monroe and to the founding of the IHMs. The journey marked another profound transition: Theresa set aside her African-American identification and established herself in the white community. She was able to do this because she was a light-skinned person moving into an area where she was largely unknown. Only in the last years of her life did she discuss her early life with the sisters. Her background, however, was known to the bishops she dealt with and doubtless to the Redemptorist priests who had been her mentors even from the Baltimore days.

The first decade of Theresa's life in Monroe seems relatively peaceful and productive, although the growth of the Congregation was rather slow. Father Gillet left Monroe in 1847 and was followed by several Redemptorist priests, most significantly Egidius Smulders, who maintained a constant and energetic support of the Congregation. This tie with the Redemptorists was crucially important for Theresa, who had become imbued with Alphonsian spirituality [St. Alphonsus Liguori was the founder of the Redemptorists.] and who saw that spirituality as essential to the charism of the IHM Congregation.

In 1855, the Redemptorists withdrew from the Detroit Diocese, a decision of serious consequences for Theresa and the Congregation. Bishop Peter Paul Lefevere was enraged at the withdrawal; he attempted to eliminate all Redemptorist contacts with the IHM Sisters and to terminate the influence of Alphonsian spirituality. He appointed a diocesan priest, Edward Joos, Superior and Director of the Congregation, replacing Mother Theresa as the actual Superior.

Theresa was most dissatisfied about this deprivation of the Congregation's identity and autonomy. When, in 1858, an opportunity arose to return to Alphonsian influence by accepting a mission in the Philadelphia Diocese of Redemptorist Bishop John Neumann, she eagerly pursued the project. Bishop Lefevere approved, and in 1858, Theresa traveled to the town of St. Joseph in the Choconut Valley, Susquehanna County, to establish the first Pennsylvania mission. However, when Theresa attempted to open another Pennsylvania mission in 1859, Bishop Lefevere refused permission. Theresa persisted in seeking permission, also incurring the Bishop's ire by her assertive insistence that she needed to travel to Pennsylvania to settle a pressing concern at St. Joseph's. After a particularly stormy meeting with Father Joos, Theresa was deposed from office by Bishop Lefevere and ordered to leave the diocese permanently for Pennsylvania.