"The command Be ye perfect is not idealistic gas. Nor is it a command to do the impossible. He is going to make us into creatures that can obey that command" (C.S. Lewis in Mere Christianity). C.S. Lewis had a way for stating perfectly the answer to almost every fear or concern pertaining to Christianity and Christian living. In this case, he was talking about the ongoing struggle within every Christian's life to try to be like Christ. One fundamental truth to our lives is that we are sinful and are incapable of ever coming close to the perfect standard God holds us to. However, the other fundamental truth to our lives is that God had mercy on us and loved us enough to send his Son, Jesus, to die for us and make us perfect through Himself.
If we are made perfect through Christ's sacrifice, then why should we even try to do good, be good, and abstain from all kinds of evil? The answer can be found in John 15:4-6, "Abide in me and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing. If anyone does not abide in me he is thrown away like a branch and withers; and the branches are gathered, thrown into the fired, and burned." Here Jesus says that if we are truly in Him, we will produce the fruits that make us perfect in the eyes of God. Apart from Him, even the whitest of our deeds cannot brighten the deep black covering that is our sinfulness.
Some non-Christians will point to the awful deeds of people who claim to be Christian, and to that we can say that they are indeed bad examples of Christian behavior. We must also remind them, though, that even though we are Christians, it does not mean that we will no longer sin. Some Christians do and have done atrocious things to others, but that does not mirror Christ. If they repent and change their behavior, that mirrors Christ in their lives. For if Christ is in us, while our sinful condition keeps us from being perfect on our own, Jesus can fill us with grace and righteousness when we come to Him for forgiveness. It is this righteousness and grace, given to us by Christ, that makes us perfect; not what we do.
On the point that there are some Christians who do not model good Christian living, it is quite easy for some of us to look down on them as well and say, "well they do not represent all of us Christians; I am a much better Christian than they." Even if we do not say this, almost all of us have thought it at some point or another. This is where we too fall into more sin and danger; because even if our deeds appear to be "good enough" to be considered Christian to the outside world, our thoughts and attitude has just fallen into sin and we are just as damnable as those we were looking down upon.
How then are we to respond to this argument? The only answer is to be honest with the person and quote Romans 3:23, "for all have sinned and fall short of the glory of God." Even Christians sin, and not all Christians are far along enough in their faith life to open themselves completely to Christ; for while it is God who does all the work associated with our sanctification, we do have the option to refuse Him. Receiving Christ as your Lord and Savior does not automatically give you the faith and conviction on someone like Peter or Paul; that kind of faith takes development and nurturing.
I should rephrase my answer to the question in the previous paragraph; I should not have said that the answer I have is the only answer, for there is another: compliment the person for their observation and thank them for holding Christians to such a high standard, because that is what we are! God Himself holds us to such a standard; it is only right for the rest of the world to do so as well. In Luke 11:36 Jesus says, "If then your whole body is full of light, having no part in dark, it will be wholly bright, as when a lamp with its rays give you light." What Jesus is saying here is that His presence in us will illuminate us to the world so that they can see the everlasting light (Jesus Himself) within us. Just as when a lightbulb or lamp start to die or get dirty and are easily noticed by any person who walks into the now dim room, so it is when a Christian allows their light to fade.
The lesson, then, is that God wants all of us, not just part of us; and unless we give Him all of ourselves, we will not be able to be the light God expects us to be. However, once we give ourselves over to Christ completely, our lives will be turned upside down and God will make us perfect. After we have been made perfect in His sight, nothing can separate us from Him. No matter what we do, no matter how much our sin convicts us, He still loves us and will forgive our repentant hearts. As Paul writes in Romans 8:38-38, "For I am sure that neither death nor life, nor angels nor rules, nor things present nor things to come, nor powers, nor height of depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord."