J. Gresham Machen was the Presbyterian giant who taught at Princeton during the time of B.B. Warfield—another giant.
During that era, Protestant Liberalism "ruled the roost" of almost every seminary in America. It was, as usual, presented as "the thinking man's version of Christianity." Those who did not "line up" with it were subjected to various insults, disparagements of intellect, and all the usual pejoratives. Machen had gone through the virtual loss of his own Christian faith when doing doctoral work in Germany and had had to cognitively fight his way back. Of course, the same thing goes on for students today. Only the names have changed.
This little book is the tour de force against Protestant liberalism. Doctrine-by-doctrine Machen contrasts what Protestant liberals held as opposed to what Christianity has always held (I am here using secularist Walter Lippman's phrases: "Whatever else liberalism is, Dr. Machen has demonstrated that it is certainly not Christianity.")
When Machen is finished, Protestant liberalism lies as a pile of powder on the floor! This simple, short cognitive disembowelment of his opponents' position has never been answered and probably never will be.
A paradigm of polite polemics, it is to every Christian's good to read it. As previously mentioned, the characters and names change, but historical Christianity is almost always attacked by the clergy of a given era. During the beginning days of the Darwinian attack, one liberal published his book that characterized Christianity as "basically evolutionary." During the time of revolutions, Christianity was re-formed by liberals into a "religion of revolution, liberation." During the time of the sexual revolution in America, some liberals described Christianity as primarily a means to "sexual liberation." And so on and so on.
And during the close of 19th century and early years of the 20th, the American Presbyterian church was under withering attack by theologians who substituted social categories for the Gospel. Machen arose to answer that attack, to show that—completely aside from whether a person was a Christian—he or she could examine the evidence and come to the conclusion that Christianity is a message rooted in real historical events, a religion that centers not on morality but on the historical news (and evidence!) that God had become man in Jesus Christ and died an atoning death for human sin, and that the sinner was offered complete forgiveness in embracing that announcement St. Paul calls "the Gospel." Majestic!