Somebody who has not entered into the true sheepfold by repentance and faith and conversion to Jesus Christ is a stranger. Somebody who teaches false doctrine which is not in the Scripture is a stranger.
Truly saved people, instinctively almost, will not listen to a person, a religious teacher, who denies the Bible, who denies the incarnation of Jesus Christ, his suffering and death on Calvary, his resurrection, his teaching about the need for repentance and faith, and an experience called conversion, or the need for the new birth. The stranger could be dressed like a shepherd, but the true sheep is not fooled because they know the authentic message of an inspired Bible and the work of Christ and the need of salvation.