[1] After graduating from Central High School, he attended St. Joseph's College and St. Charles Borromeo Seminary, all in Philadelphia.

In 1894, many parishioners started a new independent parish, Immaculate Heart of Mary, with Kolaszewski as pastor; Horstmann excommunicated all of them.

After removing Reverend Casimir Zakrekac as pastor of St. Vitus Parish in Cleveland, he faced violent protests.

On September 22, 1907, 5,000 Polish protesters marched on Horstmann's residence, demanding Zakrekac's reinstatement and home rule for St. Vitus.

[5] As bishop, Horstmann erected 22 ethnic parishes, but faced opposition from nationalist schismatic groups, including the Polish National Catholic Church.

[2] He was described the Catholic Encyclopedia as "a zealous pastor of souls, a wise and prudent ruler, a fearless defender of truth.