“They [the children of God] do not misuse their liberty, for from the heart they aim to serve others and rejoice in doing so. It is not that the new man is required to serve his neighbor, any more than that a tree is required to bloom, or 3 plus 7 are required to equal 10, or the sin sun required to shine.” — Martin Luther (WA Tischreden VI, 153.)

Taken from Gustaf Wingren “Luther on Vaocation” (99). Once again you’ve got to love Google and Wikipedia: I typed in ”
WA Tischreden” and, voila, my ignorance was dispelled.