It is a way of passive aggressively, or even jovially saying hello to someone through religion.
Common responses to this can be "i am no friend of the lord". Worry not you can inform them that the lord still loves them.
Hello Friend of the Lord, how are you on this glorious of days.
Hello Friend of the Lord, how are you on this blessed of days.
It has many uses and can be used to reference someone that either you like or dislike.