[1] Seitz was ordained to the priesthood for the Diocese of Dallas by Bishop Thomas A. Tschoepe at Saint Joan of Arc Church in Okauchee, Wisconsin, on May 17, 1980.

In 1985, Seitz attended Saint John's University in Collegeville, Minnesota, where he was awarded a Master of Liturgical Studies degree that same year.

[1] He also worked as an instructor at the Christ the Servant Institute in Dallas in 2001 and interned at the National Catholic Bioethics Center in Philadelphia, Pennsylvania, during the fall of 2002.

On July 18, 2017, Seitz issued a pastoral letter on immigration in which he said that "elected leaders have not yet mustered the moral courage to enact permanent, comprehensive immigration reform" and praised the efforts of the "heroic individuals, families, pastors, religious, parishes and institutions that spend themselves in service to migrants and refugees" and campaign "against the militarization of our border".

He expressed concern for immigrant families who fear separation and for law enforcement officers who "put their lives on the line to stem the flow of weapons and drugs" but are "troubled in conscience by divisive political rhetoric and new edicts coming from Washington, D.C." He condemned profit-based immigrant detention centers, the hostility shown to asylum seekers, and "the disparagement of our Muslim brothers and sisters".

[7] In February 2022, Seitz condemned a lawsuit filed by Texas Attorney General Ken Paxton against Annunciation House, a Catholic charity in El Paso.

When this system begins to shape our public choices, structure our common life together and becomes a tool of class, this is rightly called institutionalized racism.