It is saying that when someone in high power (such as a priest) is corrupt or wrongdoing, then people will still follow him blindly, not seeing that he is wrong. We put too much trust into people of higher power and need to exam them to make sure they have good intentions. the shepherd and the flock are symbols. The shepherd is the higher power, and then flock is the people. The flock of sheep will follow the shepherd no matter what, as we humans often do. I hope this helps :)

For if a priest be foul in whom we trust no wonder that a common man should rust; and shame it is to see - let priests take stock - a shitten stepherd and snowy flock. thanks It's saying: If a holy man such as a priest is corrupt, and he is a man that we put our trust in, it's no wonder men fall and fail so hard. It's sad that the people blindly follow another man, who even though is a priest is also corrupt. The people are the flock, the priest is the Sheppard.