Jesus' greatest enemy was a purity movement.
The Pharisees were a sect of Jewish leaders and teachers whose main goal was to purify Israel of its sin so that God would deliver Israel from Rome. In the law they saw the fundamental promise: "If you obey you will be blessed and dwell in the land." And so they took it upon themselves to be obedient and to enforce obedience across their small country.
The problem was that they thought they were the obedient ones.
Swallowing a Camel
The Pharisees were meticulous in following the law, while actually missing the grand intent of the law. It was not only to be pure and blameless, but also to be loving and good. That is why Jesus said that they would "strain a gnat, but swallow a camel." They neglected mercy and justice to their fellow countrymen.
But we ought not to just think that these men were more evil than any of us. They just had it in their mind that they were worthy of being blessed, and they tried to root out any people that would withhold the blessing from the nation—prostitutes, tax collectors, and "sinners."
The Fundamental Question
And that is the problem with all purity movements, whether they are doctrinal, methodological, or moral. The leaders see themselves as part of the solution, and act as referees in their culture—sitting in judgment of those who, in their minds, are withholding blessing from their nation. And so they have to be merciless and judgmental, because their fundamental question is "What must we do to be blessed?" instead of "God has already blessed us—what must we do to love?"