Why does Jesus says to those who do miracle that he never knew them?

Jesus was trying to explain that just because a person does good works, does not mean that is all they need to enter into God's kingdom. Jesus came to earth as a model for us to follow, he preached God's message about his kingdom to rule in the future to everyone-which we should be doing now. There are many preachers and nuns who claim to devote themselves to God, but are they going door to door preaching to people about God's kingdom as Jesus did? Are they truly worshiping the true God with spirit and truth? Also some are more concerned about being righteous for the "look" of it, to make them feel better about themselves. -this is not genuine either.