When the United Presbyterian Church in the USA merged with the Presbyterian Church in the United States there were 3,131,228 members. Statistics shows steadily decline since 1983. (The combined membership of the PCUS and United Presbyterian Church peaked in 1965 at 4.25 million communicant members.[49])
Reference 1: Membership and pct change.
Reference 2: 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 and 2015.
Source: http://en.wikipedia.org/wiki/Presbyterian_Church_(U.S.A.)
Reference 2: 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 and 2015.
Source: http://en.wikipedia.org/wiki/Presbyterian_Church_(U.S.A.)
