What was Big Willy Shakes going for?
King Phillip is saying that we cannot "play fast and loose with faith," or to put it in laymen's terms: we can't go around being unreliable when it comes to Important Things.
That all sounds well and good, but our guy's not exactly practicing what he's preaching. Sure, he might be condemning people who are fickle, but he's one of those people. Constance points that out and he can't stand it. But don't worry, he'll change sides again before the play is up.
So what does it actually mean to "play fast and loose" with something? Shakespeare didn't think this one up all by himself. Before Shakespeare was even alive, this was the name of the game. Literally.
People would play fast and loose (or Pricking the Garter) by placing two belts in a coil on the ground. Someone would be asked to find the true center of the loop by placing a stick in it. The person playing would have to get a stick to hold fast to the belt when it came loose (because it was pulled on either end).
Challenge accepted.
But, this wasn't as easy as it seemed. The operator (read: scam artist) would pull the ends so the stick would fall. He usually had a trick belt or a faulty coil so the stick would fall off immediately. So he would always win. So no one could play "fast" and "loose."
People played this game back in the Middle Ages, and when Shakespeare was roaming around. He wasn't the first to use this phrase, but he loved it. He used it again in Love's Labour's Lost and Antony and Cleopatra.