InOBU, here is a good link to A man for all Seasons

The quote you mentioned doesn't appear in the "Memorable Quotes" section for that film, though I suspect that Moore (played by Paul Schofield) actually quoted it to Richard Rich, a trimmer rather than a zealot; or perhaps to Thomas Cromwell, I don't remember the film well enough.

I do remember my favourite quote from the film though. Richard Rich has just finished giving evidence against Moore.

Moore: "One moment. I notice that you wear a chain of office, now. May I see it ?" He examines the chain closely "A red dragon ?" Rich: "I am Attorney-General for Wales."
Moore: "Ah! Remember, Richard that it shall not profit a man to gain the whole world, if he loses his soul. But Wales ?!"