At the beginning of the twentieth century Franklin Square had no organized religious congregations.  Most of the population was devoutly Catholic; the remainder largely Methodist.  Catholics traveled to neighboring villages for mass on Sundays and Holy Days.  A few attended Our Lady of Loretto in Hempstead.  Most Franklin Square Catholics were members of St. Boniface’s in Elmont.  This church drew many Franklin Square people because sermons, devotions, and confessions were conducted in the German language.  A small number of local people attended Holy Ghost Church in New Hyde Park, another German-speaking congregation founded in l893.  Throughout this period the Franklin Square area fell under the jurisdiction of the Diocese of Brooklyn. 

By the year 1908, Franklin Square had grown enough to warrant a church of its own.  Local people were tired of going to Hempstead or Elmont  for Mass, and the leading Catholics of Franklin Square began to discuss the feasibility of organizing a parish.  In the spring of 1908, Nicholas Kreischer, Peter J. Herman, August Kalb, and Jacob Hoffman petitioned the Bishop of Brooklyn, Charles E. McDonnell, for help.  The bishop was sympathetic to the petition, but there were many similar country districts in need of a church, and not all of them could be served.  Finally the bishop gave his permission for a church to be organized, providing Kreischer, Herman, Kalb, and Hoffman would guarantee that it be self-supporting.  The four men promised that the Franklin Square Catholic community would cover all the necessary costs.  After securing permission from the Bishop, the Franklin Square area became an out-mission of Our Lady of Loretto in Hempstead.  As a mission, Franklin Square did not have a resident priest.  The spiritual leader was Father Farrell of Our Lady of Loretto. 

Immediately, local Catholics began to discuss potential names for the future parish.  Only one named was seriously considered, for "The new parish was founded under the patronage of St. Catherine of Sienna, a designation undoubtedly influenced by the fact that the first name of the wives of the four men (Kreischer, Herman, Kalb, and Hoffman) was Catherine!  Interesting, too, is the spelling of the name "Sienna," usually spelled with one "n" (Siena).  The founders chose a German spelling, one which endures until the present day. 

The first Holy Mass to be celebrated in Franklin Square was scheduled for Sunday, July 5, 1908.  It was decided to hold Mass in the Kinsey Hempstead, a fine ten-room Victorian mansion on the east corner of Hempstead Turnpike and Scherer Boulevard.  The ample front parlor was prepared for Mass.  Rose Landgraft provided an altar cloth and fresh flowers; Cecilia Kalb played a large piano which was in the parlor; young Andrew Hoffman served as altar boy.  A choir consisting of Christian Adams, his sister Alma Reiss, Joseph Kreischer, and Joseph Hoffman stood on the stairs.  The Mass was celebrated by Father Farrell (1867-1930) from Hempstead.  Families attending included that of Jacob Hoffman, John Hoffman, the Volkommers, Kalbs, Schucks, Adams, Steins, Raths, Gaynors, Landgrafts, and Stevens.  There were perhaps sixty or seventy people in all.  The  first child baptized was Herbert Hoffman, son of Mr. & Mrs. Joseph Hoffman.  The ceremony took place in the Hoffman home.