From Luther’s sermon for the second Sunday after Easter.
How is he known of the Father? Not with an earthly, but with a heavenly, knowledge. Of that we have spoken more fully before, and the substance of it is this: Christ recognizes us as his sheep, and we recognize him as our shepherd. Now, we have heard what a good shepherd is, and also who the weak sheep are. He knows us to be such sheep as are weak, sick and broken. That is: It does not make any difference in his regard for them that they are weak and sickly, and he does not despise and reject them on that account; but he pities and heals them, even though they be so diseased that the whole world concludes they are not his sheep. Such is the world’s knowledge, but that is not the way that Christ distinguishes them.
He does not look upon their condition, but looks to see whether they are sheep, whether they may be designated sheep. He looks at the sheep, not at the wool.