Complex sentence: Upon his return to New York, Hayes became an assistant pastor at St. Gabriel's Church on the Lower East Side, where he served under its pastor, John Murphy Farley (whom he would later succeed as Archbishop of New York).
Split into simpler sentences: Upon his return to New York, Hayes became an assistant pastor at St. Gabriel's Church on the Lower East Side, where he served under its pastor, Msgr. John Farely (whom he would later succeed as Archbishop of New York).