Do not judge, or you too will be judged. For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you. Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye? How can you say to your brother, "Let me take the speck out of your eye," when all the time there is a plank in your own eye? You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother's eye. (Matthew 7:1-5)
This verse helped changed my life dramatically in college. I was in a very judgmental stage in my life; what I like to call the "Frank is Perfect" stage. I remember vividly a non-Christian friend of mine quoting me this verse, and asking me to reexamine myself. My first reaction was "Who does he think he is quoting me, Mr. Christian, a bible verse??" It took awhile for his words to sink in, but they finally did. I no longer am so quick to judge people, but rather try to understand where they are coming from, or why they do the things they do. I no longer view myself as "Mr. Perfect" but as "Mr. Sinner" - a man in desperate need of God's grace.