Cas was one of the angels that fell for Zachariah's lies and thought he was suppose to stop the seals from being broken. Cas didn't know the truth until 4.20 The Rapture and he was going to tell Dean, but Zachariah and others took him (leaving just his vessel, Jimmy Novak). Cas experienced some of "heaven's persuasion" and so kept quite until 4.22 Lucifer Rising when he decided to finally tell Dean the truth about Lilith being the final seal.