Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Sir Thomas More
More is a man of an angel's wit and singular learning; I know not his fellow. For where is the man of that gentleness, lowliness, and affability? And as time requireth a man of marvelous mirth and pastimes; and sometimes of as sad gravity: A Man for All Seasons. Robert Whittinton
He was the person of the greatest virtue these islands ever produced. Samuel Johnson
What's up with the epigraph?
The epigraph basically functions as a résumé for Thomas More. To hear these two dudes tell it, More's pretty much the greatest dude in all of England, if not the world.
Some historical context wouldn't hurt here. Robert Whittinton was a historian and writer who lived at the same time as More—he also probably knew the dude, so his opinion may count more than most. Samuel Johnson wouldn't be born for another hundred-some years, so he's mostly talking about the legend of Thomas More. But Johnson is so legit that you ought to take his word on this one, too.
Of course, Whittinton's quote also gives us the title of the play: A Man for All Seasons. This just further emphasizes More's greatness. It doesn't matter if it's winter, spring, summer, or fall; it doesn't matter if you're trying to win a court case, discuss the finer points of falconry, or kick back a few glugs of wine—Thomas More's the man you want at your back.
Man—that makes the whole decapitation thing such a shame, huh?