J. Gresham Machen:
Things that are false will accomplish a great many useful things in the world. If I take a counterfeit coin and buy a dinner with it, the dinner is every bit as good as if the coin were a product of the mint. And what a very useful thing a dinner is! But just as I am on my way downtown to buy a dinner for a poor man, an expert tells me that my coin is a counterfeit. The miserable, heartless theorizer! While he is going into uninteresting, learned details about the primitive history of that coin, a poor man is dying for want of bread. So it is with faith. Faith is so very useful, they tell us, that we must not scrutinize its basis in truth. But, the great trouble is, such an avoidance of scrutiny itself involves the destruction of faith. For faith is essentially dogmatic. Despite all you can do, you cannot remove the element of intellectual assent from it. Faith is the opinion that some person will do something for you. If that person really will do that thing for you, then the faith is true. If he will not do it, then the faith is false. In the latter case, not all the benefits in the world will make the faith true. Though it has transformed the world from darkness to light, though it has produced thousands of glorious healthy lives, it remains a pathological phenomenon. It is false, and sooner or later it is sure to be found out.Faith in faith never saved anyone. Theology matters.
Such counterfeits should be removed, not out of a love of destruction, but in order to leave room for the pure gold, the existence of which is implied in the presence of the counterfeits. Faith is often based upon error, but there would be no faith at all unless it were sometimes based upon truth. But if Christian faith is based upon truth, then it is not the faith which saves the Christian but the object of the faith. And the object of the faith is Christ. 
1. J. Gresham Machen, Christianity and Liberalism (Eerdmans, 2009), p. 120.