The Shorter Writings, 217-227. In it, Machen lays out the case for the renewal of Presbyterian Reformed witness in the midst of the PCUSA's tolerance for heresy in its pulpits. If you are interested in Machen's writings you can read Christianity and Liberalism (his best book for today’s audience) or SelectedShorter Writings, from which this is selected.
"If [The Presbyterian Church] stands for Christ, it will do the work which he has put it into the world to do. And that work is the work of witness-bearing. "Ye shall be my witnesses," said the risen Christ, according to the book of Acts; and these words are really a correct summary of New Testament Christianity from beginning to end. New Testament Christianity is not a life as distinguished from a doctrine, or a life which has doctrine as its flower and fruit, but - just the other way around - it is a life founded upon a doctrine. It is a life produced not merely by exhortation, mot merely by personal contacts, but primarily by an account of something that happened, by a piece of good news, or a gospel. The apostles set forth the great event after it had occured - they said "that Christ died for our sins according to the Scriptures, and that he was buried, and that he rose again the third day according to the Scriptures." The Savior himself in the days of his flesh proclaimed the same great event by way of prophecy: "Repent ye," he said, "for the kingdom of heaven is at hand," and "The Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many." But whether the event was in the past or in the future, both apostles and Jesus proclaimed an event; Jesus gave to his disciples a message in which he was to be offered as the Savior of the world.But if so, it is important above all else to get the message straight. When a witness takes his seat on the witness stand, it does not make much difference what the cut of his coat is or whether his sentences are nicely turned. The important thing is that he tell the truth, the whole truth, and nothing but the truth. So it is with that witness stand which is called the pulpit. It does not make so much difference about the manner in which the message is delivered; there can even be charity for imperfections in the spirit and motives of the messenger; but what is important above all things is that the content of the message should be true." [217-218]
Many persons seem to regard the duty of witness-bearing as a merely personal or individual thing…It is really quite impossible to be a member of a body and evade the responsibilities involved in membership…It is not merely this one preacher who is saying these things so diametrically opposed to the gospel of Christ, but it is the whole Presbyterian church. The constitution of the church plainly regards the preacher as a representative of the whole body, as a man who sets forth the system of doctrine taught in the Word of God, and it plainly gives the courts of the church power to remove any preacher who is preaching what is contrary to that. But with power always goes responsibility. The whole church is saying to many a little one, especially now that the issue has once been raised: “This that you hear in the First Presbyterian Church of New York is the way of salvation; heed the exhortation, and you will be saved.” And the responsibility of the whole church is also the responsibility of every individual member; the government of our church is democratic, and democracy involves responsibility for the individual. Every individual member of the church – to say nothing of ministers who are members of church courts – has a vital responsibility for what is done in the pulpits and still more plainly in the agencies and boards. Individuals must witness for Christ, but the church must also witness in its corporate capacity, and no individual is walking uprightly according to the truth of the gospel if he acquiesces in a corporate witness that is false.” [220-221]
“If a man may “Interpret” a perfectly plain confession of faith to mean its exact opposite, what is the use of having any confession at all?” 
“The public testimony of this preacher, and of the many preachers like him, is diametrically opposed to the gospel of Christ and is leading men away from the true Savior of their souls; it is producing a confidence in human goodness, in human ability to obey the commands of Christ, which it is the first business of the Christian preacher to break down. For our part, we feel compelled to relinquish all confidence in such goodness and to trust in the crucified Savior alone.
We certainly do not wish to “split” the church; on the contrary, we are working for the unity of the church with all our might. But in order that there should be unity within the church, it is necessary above all that there should be sharp separation of the church from the world. The carrying out of that separation is a prime duty of the hour. Those who try to save men by the ethical principles of a Jesus whom they have reconstructed through rejection of the New Testament witness should form one body; those who glory in nothing save the cross of Christ should form another body. And that latter body is the Christian church. What our Savior demands of us above all else is faithfulness. He has placed us in the world as witnesses, and the supreme duty of his witnesses is that they should testify faithfully to him.
The Presbyterian church, we are convinced, is still predominantly Christian; it would stand for Christ if it knew the real meaning of the hostile propaganda which is now attacking the center of its life. At such a time clearness is demanded of every Christian man; the hour for merely pleasant words is over; love demands the plain speaking of the truth. We are witnesses; and if we are faithful to our sacred trust we must witness truly, whether men hear or whether they forbear.
What shall our decision be? Shall we transfer our allegiance to another gospel? Or shall we, by the help of God’s Spirit, be faithful to the Lord and Savior who bought us with his precious blood? [226-227]