“Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.” Hebrews 13:2 (KJV)
A brother rings the bell at a bad hour. The child has just fallen asleep. The kitchen is not clean. The morning was hard. It would be very easy to answer the door in a mood — to give a smile that does not reach the eyes and a conversation that does not last five minutes.
Hebrews warns the household otherwise. Some have entertained angels — angels — without knowing it. The interruption at the bad hour was, in scripture, sometimes the Lord Himself. The tired kitchen was, sometimes, the room where the good thing of the whole week happened. We do not always know which knock is which. So the household treats every knock the same: as if it might be Christ at the door.
This is the whole of Christian kindness in one small habit. Not a technique — a posture.