Everyone who has made even the smallest start in Christian knowledge realizes that it is both dangerous and destructive to a Christian’s soul to join a sect that publicly rejects God’s Word and denies all the mysteries of the Christian faith. However, if the heretics of a particular sect are not so coarse and obvious, then many think there is no harm in joining it, since it still appears to subscribe to the principal Christian doctrines. Such heterodox fellowship often appear to be especially friendly, humble, and zealous, and many suppose that joining such a sect might result in even more blessing and edification than the orthodox Church could provide. To be sure, the truth as it is preached in the orthodox Church often seems to wear a rough garment.
God Grant It: Daily Devotions from C. F. W. Walther p 382