Recommended by Dr. Stephen Nichols' podcast (5 Minutes in Church History) on Machen (1881 - 1937).
Source: http://www.jstor.org/stable/1018160?seq=1#page_scan_tab_contents
This 10 page work deals eloquently in matters of church and liberalism with simplicity.
Machen immediately contrasts post-modernism influence in churches with the "primitive" church (the true church). Essentially, the primitive still is, while the new age was 2000 years ago in Plato's Republic (we gave into bureaucratic control). The focus of the entire article is always gospel centered (The need of Christ).
From tyranny to tyranny, this is the legacy of liberty by human attempt without God.
Machen then emphasize the few distinctions of a true church:
1. Radically doctrinal
2. Radically intolerant
3. Radically ethical
This leads to attack from outside as well as from within of the church. But they are nothing new, just in different forms throughout history. The preservation of the true church's characteristics is a covenantal kind.
Then there's warning against involving church with non-christian programs (placing Bible in schools in order for students to have moral vs. the redemptive core of scripture) and partnership with the state in politics in carnality - The church's weapons against evil are spiritual, not carnal.
At the end, under "What the church should do", Machen described the kind of good churches one would expect: not by numbers, human authority, etc. The message/sermons of a true church is a paradoxical one in secular social ethics. In the end, it is a matter of destruction and life.