Kwasniewski was also a signatory of a letter in support of the four Dubia cardinals. In April 2019, Kwasniewski was among 19 Catholic scholars and clergy who signed a 20-page open letter making a formal accusation of heresy against Pope Francis. In addition to being a teacher and writer, Kwasniewski has enjoyed a parallel career as a choirmaster, cantor, and composer of about 150 works of music. After studying composition and conducting with Roy Horton and voice with Carol Horton at Delbarton School in Morristown, New Jersey, Kwasniewski became assistant choir director at Thomas Aquinas College in Santa Paula, California (1990–1994). He served as director of the Schola Cantorum at Old St. John's Catholic Church in Silver Spring, Maryland (1994–1998), director of music at the International Theological Institute in Gaming, Austria (1999–2006), and founding choirmaster and schola director at Wyoming Catholic College (2007–2018). Kwasniewski's Mass settings, motets, hymns, carols, antiphons, and acclamations have received numerous performances from 1990 to the present. His Mass settings include "Missa Spiritus Domini" (1994), "Missa Spe Salvi" (1995, rev. 2012), "Missa Brevis" à 3 (1997), "Missa Hereditas Mihi" à 3 (2016), and "Missa Honorificentia Populi Nostri" (2017). His "Seven Mandatum Antiphons" (2010) were dedicated to Arvo Pärt on the occasion of his 75th birthday; the maestro personally accepted the dedication. Many of these works can be found in his book "Sacred Choral Works," which was highly acclaimed when it first appeared in 2014. Ensembles that have performed Kwasniewski's choral music include the Ecclesia Choir (dir. Timothy Woods), the Vittoria Ensemble (dir. Rick Wheeler), the Scottish ensemble "Cantiones Sacrae" (dir. Graeme Adamson), and Cantus Magnus (dir. Matthew Schellhorn). Kwasniewski has also written a three-movement wind quintet "Fire and Flame," premiered by the Antero Winds in 2012; an "Ave Maria" for SATB, winds, and strings; a violin sonata; organ accompaniments for English plainchant; and pieces for brass quartet and string quartet.