Now you might ask how you can know that we have received this child in our heart
through true faith as our own and how we can be sure that this birth is effective
for us. Therefore we want to say a little about this token, which enfolds this
child, which is surely there and results in external works from the heart. This sign
is nothing other than that we also receive people to ourselves so that we take on
and clothe ourselves in our neighbor’s flesh and act toward him as God does to us in
this Christ. That is also the sign of a spiritual birth and the making of a
spiritual man. For it is in this manner we are born also unto each other.

The Scripture calls our neighbor “our flesh.” For Isaiah says [chap. 58], “If you
see someone naked then clothe him and do not turn away from your own flesh.” He says
what the other person is. He is our flesh. For God wants no one to despise another
person. Yes, the poorer he is, the more I should receive him in no other way than as
if he were my own flesh and blood, my own body. For Christ has born our flesh a even
though our flesh was full of sin and filled with every trouble and misfortune.

Luther, Sermon for Christmas Day – Early Service