J. Gresham's "Christianity and Liberalism" addresses the topic of heretical, revisionist teachings and the damaging effect these bring within Christian churches. By 'liberal' Machen isn't referring to political liberalism but rather the introduction of revisionist, distorted views of the Scripture, such as questioning Biblical innerancy and the historicity of the Scripture. What's remarkable is that Machen wrote this back in the 1920's, but all of the liberal heresies that he addresses are just as much alive and well today. It isn't uncommon to find the pastor at a supposedly "Christian" church that denies important Biblical truths such as the virgin birth or even the resurrection of Jesus Christ. I'm reminded of a message I heard recently from pastor Phil Johnson, in which he rightly observed that the greatest threat to Christianity today isn't the raving atheist on the street, but rather the internal corruption of liberal teachers from within the church itself (the quote comes from Johnson message on the topic of pastoral shepherding, that's available here)
Machen draws the clear distinction between "Christianity" and "liberalism", identifying there is no such thing as liberal Christianity, but rather there is orthodox, confessionally-centered Biblical Christianity, at the core of which is gospel-centered faith in repentance and belief in the resurrected savior Christ Jesus, and any supposedly “christian” religious approach that detracts from that, or throws doubts into the historical claims of the Bible, particularly those that undermine the fundamental, historical tenants of Christianity, Machen rightly distinguishes as something other than Christianity, as another religion.
Machen addresses numerous liberal arguments made about the historicity of Scripture, miracles, teachings such as of the "universal fatherhood of God", attacks on the Pauline epistles, the work and person of Christ, the importance of doctrinal standards in the church, etc. It's a fascinating and relevant read for laypeople. Consider Machen's closing remarks, "There are congregations, even in the present age of conflict, that are really gathered around the table of the crucified Lord; there are pastors that are pastors indeed. But such congregations, in many cities, are difficult to find. Weary with the conflicts of the world, one goes into the Church to seek refreshment for the soul. And what does one find? Alas, too often, one finds only the turmoil of the world. The preacher comes forward, not out of a secret place of meditation and power, not with the authority of God's Word permeating his message, not with human wisdom pushed far into the background by the glory of the Cross, but with human opinions about the social problems of the hour or easy solutions of the vast problem of sin. Such is the sermon... Thus the warfare of the world has entered even into the house of God, And sad indeed is the heart of the man who has come seeking peace." (p. 179-180)
Reading Machen should spawn good questions for all Christians, and one should not be afraid to bring these up with your pastor. Ask the pastor/elders if the church is confessionally-centered, if it adheres to the historical creeds (such as the Apostles creed/Nicean creed) and the truths defined therein. Ask the pastor if the church adheres to the inerrant authority of the word of God. Ask if the church believes in and teaches the miracles of Christ, and especially the resurrection. If you don't get clear answers on any of these, then give careful consideration if you're actually a part of a Christian church, or instead within a liberal false-church that actually has no part of the true body of Christ.
"Christianity and Liberalism" is available at most online bookstores, but since it's in the public domain its also freely available electronically at sites like this.
You can also hear a free MP3 recording of the book here: http://reformedaudio.org/machen.html
For another example of the detrimental damages done by liberalism in the Christian church, also check out the recent testimony of a German Christian named Anita Kupferman and her exposure to “higher critical methodology” in some of German “theological” schools (her testimony can be read in HeartCry magazine, available for free here)