"That was another mystery: it sometimes seemed to him that venial sins—impatience, an unimportant lie, pride, a neglected opportunity—cut off from grace more completely than the worst sins of all. Then, in his innocence, he had felt no love for anyone: now in his corruption he had learnt."
The Power and the Glory is a look at Catholicism from the prospective of a Catholic. Greene's whisky priest is a bad man. He has not obeyed the laws of God, but he still has the power to absolve others from their sin. His compassion compels him to hear the confessions of the spiritually abandoned Mexico, and forces him into a life of hiding and fear.
The 'bad guys' are the police who seek to kill the whisky priest, but they are really no different than he is. They are officials who break the law, yet are fighting to eliminate all religion on the basis that this will give the children of Mexico a better life than their ancestors.
The difference between the two is what their purpose. The whisky priest is an instrument of God, but still cannot escape his own sin. The officials are sincerely striving to do right, unfortunately it is all in their own eyes.