Meaning of 'If gold rusts, what then can iron do?'

Geoffrey Chaucer's famous quote appears in The General Prologue to his best known work, The Canterbury Tales.

The modern and most commonly quoted version is: 'If gold rusts, what then can iron do?'

However, in the original Middle English the text is: 'That if gold ruste, what shal iren do?'

In terms of its context, Chaucer tells of a poor Parson, a local priest, who lived a humble life doing good deeds in the community. The motto he lived by was this quote and Chaucer goes onto explain its meaning in the next line:

'For if a preest be foul, on whom we truste,
No wonder is a lewed man to ruste'

So, if a priest is corrupt what hope is there for the rest of us, or to use the metaphor if the most precious of metals rusts, then what hope is there for a baser metal in iron?

As such a priest should set an example of how to live to his parish as if he does not then how can he expect others to be good.