The the Sermon on the Mount in the gospel of Matthew contains a startling statement: "Therefore You are to be perfect, as your Heavenly Father is perfect (Matt. 5:48). Many have used these words to justify a perfectionist attitude characterized by a strict obsession with proper behavior and an intolerance of mistakes. But it seems that they have failed to notice the way Luke phrases Jesus' words in his account of the same sermon--"Be merciful, just as your Father is merciful" (Luke 6:36).
It would seem that some Christian perfectionists have an imperfect understanding of what the Bible has to say on the topic, because 'merciful' isn't usually the first word that pops into your head when you think of such a person. I think this is because they fail to realize that perfection in the Bible isn't about behavior. Luke doesn't say, "Do merciful deeds"; but rather "Be merciful."
Perfection is easy to achieve when it means trying to be like other 'perfect' people, pointing out and condemning others mistakes, and not focusing on the big issues that we have in our own lives (Luke 6:37-42). But it's a little more difficult when we realize that our standard of perfection is the attitude Jesus had and that good deeds are not just obeying the rules but are judged by the effects that they have on others (fruit). It's even harder to achieve when we come to understand that perfection produces behavior, and not the other way around (Luke 6:43-45).
Perfection is impossible without Jesus, "the author and perfecter" of our faith (Heb. 12:2). He's the only one who has the ability to take out our imperfect, unmerciful mindsets and replace them with a perfect, merciful mind that reflects who he is. This doesn't mean we won't make mistakes some times, because perfection is a process. Paul says that those who are perfect will realize that they aren't (Phil 3:12-15). But we can continue on nevertheless, because with Jesus as our perfection and forgiveness we have nothing to fear (1 John 2:1).