‘To put gold on the back of the holy image.’
This idiom is ancient and the concept is found in many cultures.
From ancient times, one could make merit by placing gold leaf on a holy image. Doing this showed that a person was not attached to material, but rather the ethereal.
As with most devotional customs, the idea became a contest and people with much money would make a large show, putting large amounts of gold leaf on an image to give them great ‘face’.
One very wise monk taught that this practice was wrong, and nullified any merit made.
The monk taught the people that true giving is done quietly and without thought of worldly approval or gaining face.
Anyone who really wished to make merit should put the gold leaf on the back of the image, preferably when no-one else was watching.
The giver then gained true merit.
The western equivalent might be ‘Virtue is its Own Reward’ or the Bible’s ‘Do not let the right hand know what the left hand is doing.’