"On the next day, as they were on their way and approaching the city, Peter went up on the housetop about the sixth hour to pray.  But he became hungry and was desiring to eat; but while they were making preparations, he fell into a trance; and he saw the sky opened up, and an object like a great sheet coming down, lowered by four corners to the ground, and there were in it all kinds of four-footed animals and crawling creatures of the earth and birds of the air.  A voice came to him, 'Get up, Peter, kill and eat!'  But Peter said, 'By no means, Lord, for I have never eaten anything unholy and unclean.'  Again a voice came to him a second time, 'What God has cleansed, no longer consider unholy.'  This happened three times, and immediately the object was taken up into the sky."

God is not telling Peter to abandon his piety, just abandon his need for piety.

To place these animals in front of a practicing Jewish man, then to ask him to eat, is considered an insult.  Peter would have been confused at the lack of "Religious strength" of the statement, eat.  Even though he was confused, he still had the presence of mind to remind God how important the Law has been to him. 

That is the ultimate reason for the insult, the dream, and the command - the Law is servant to God and the Godly - not the other way around.

So, Peter was told that he should use the commands of God as a lens in his world view.  The Law is meant to be used as a lens into our personal lives.

Today, look at people through God's eyes.