One may read the whole Holy Scripture through, from the first book to the last, and one will find not a single place, in which individual Christians or whole congregations are allowed to hear those teachers, or even to cultivate association with them, who, be it in much, be it in few points, teach false doctrine. But we can find hundreds of places where Christians are most solemnly commanded to stay away from those who teach false doctrine. "So you are excommunicating all other church associations!" people say to us. Not at all. We know and confess, that even in heterodox communities, insofar as in the same there are still substantial parts of God's Word in circulation, there are many dear children of God, who from a lack of knowledge [Tr. note: literally "weakness of recognition"—Schwachheit in der Erkenntniß] are in a camp where they do not belong. But it would be against God's will and foolish, if we, for the sake of these erring Christians, create an association with the false teachers and thus confirm the erring Christians in their error and the false teachers in their false doctrine. In the circumstance that those 200 men went from Jerusalem with Absalom in their lack of knowledge [Tr. note: 2 Samuel 15:17], it would not be right for the loyal Israelites to go on their part into the camp of the rebels, or even to cultivate friendly relations with them.
Our position to the efforts at church union at present is continuously much criticized. Our position is this: We lament the shredding of the church into many parties teaching diverse doctrines. This shredding is a work of the devil, through which unspeakable damage is done to the church. But how is the damage to be healed? The division arose in that false teachers emerged and Christians, instead of avoiding them, held to them. If one wishes to undo the division, there is no other way in the world other than that Christians once more isolate the false doctrines, censure it and avoid it. In order that Christians be in a condition to do this, we continuously fight the false doctrine in our magazines, from the chancel and also in private conversation, and give witness to the right doctrine. We are also prepared to do more, e.g. to gather at the so called free conferences and there, with all patience, to deal with the doctrinal differences from God's Word. To put a mere "outward association" without a unity in all articles of the doctrine revealed in the Scripture on the program; otherwise said, to negotiate how much of God's Word we can leave out, instead of how one can can come to recognition and to acceptance of the whole revealed truth—that is not fitting for Christians.
from Franz Pieper, 'Our Position on Doctrine and Practice'