"they are the current gold coin of the New Jerusalem, with the King’s own mint-mark on them"
Charles I sixpence from 1639
GNU Free Documentation LicenseCharles I sixpence from 1639 - Credit: Classical Numismatic Group, Inc. http://www.cngcoins.com

Because of the political upheavals then taking place in England, this isn’t the simple affirmation of the value of prayer it might initially seem. The New England settlements shared the currency of their colonial rulers and so their coins bore the mint-mark of the English monarch. However, when this scene takes place — just before the election of May 2nd 1649 — there was no monarch at all, Charles I having recently been executed for pursuing his own selfish ends at the expense of his country. The narrative present marks the beginning the decade-long impasse known as the Interregnum in which England was a de facto republic, and it is this which Chillingworth calls “the New Jerusalem.” He is therefore slyly insinuating that Dimmesdale’s prayers, like a coin bearing the stamp of the disgraced and decapitated king, are empty mockeries.