From time to time, our congregation has visitors and attenders from other Presbyterian churches. For many of these folks, the worship, practice, and beliefs of HPC are quite different than what they thought was a uniform Presbyterianism. "Presbyterian" no longer describes a single unified church in America for a variety of reasons. The main reason is theological differences that confessional and evangelical Christians within the mainline Presbyterian Church in the U.S. had with the direction that their mother church was taking. These confessional and evangelical Christians, who sought to hold onto the Reformed faith in the midst of what they perceived as encroaching unbelief and apostasy, separated at several points in the 20th century to form more pure expressions of Presbyterian and Reformed churches. The largest of these separations occurred in 1973 when many congregations in the mainline southern Presbyterian Church withdrew to form a new and separate denomination called the Presbyterian Church in America (a.k.a., the PCA).
For history buffs, the Presbyterian Denominational Chart of the American churches provides some context for why there are so many "Presbyterian" churches, that are "tongue-in-cheek" called the "split p's". Sadly, this is true. Presbyterians have a history of division, but at least there is also a pattern of reunion.
If you are familiar with mainline Presbyterian beliefs and practices (as expressed in the one mainline Presbyterian denomination in America, called the Presbyterian Church in the USA--the PCUSA) and the PCA is a little mysterious to you, then you might benefit from a comparison of beliefs between the PCA and the PCUSA.
Scripture and Hermeneutics [the Bible and interpretation]
Subscription to Doctrinal Standards by Church Officers
Church Polity [governing principles]
The PCUSA also publishes on its website a section called Presbyterian 101 that summarizes their core beliefs and practices. A quick comparison of the two may not reveal significant differences (because all churches strive to put their best foot forward and hide their controversial beliefs in the footnotes), but the PCUSA's positions on controversial social issues demonstrate the fundamental differences between confessional, evangelical, reformed, and presbyterian Christians on the one hand, and mainline presbyterians on the other. These differences are not merely pragmatic, but are at root theologically irreconcilable.
J. Gresham Machen, one of the leaders of an earlier group of presbyterians who separated from the mainline presbyterian church to found the Orthodox Presbyterian Church (OPC), wrote a short book clearly and convincingly demonstrating that faithful, confessional Christianity is an altogether different religion than the "liberalism" that nearly all the mainline denominations were quickly adopting in the early 20th century. Since this theological rift, known to history as the "Fundamentalist-Modernist Controversy", evangelical Christians who find themselves unable to fellowship with adherents of the faith of "liberalism" have been tearfully separating themselves from their mother churches to more faithfully serve the Lord Jesus Christ who reveals himself in his holy, inerrant, and infallible Word.