In this video by Will Goldstein, C.S. Lewis expands on the idea that those who put themselves in the hands of Christ, will after death, finally become perfect, as Christ is perfect in love, wisdom, joy and in immortality. How far this change takes place in our lifetime before death, is both uncertain and I would say, essentially up to the individual and his willingness to surrender to God. This might lead one to believe, that if this is true, all Christians should be nicer people than non-Christians. This is not always true.
First of all, some people are what Lewis calls "imaginary Christians." He seems to be implying, they are Christians in name only. There was no true conversion, and therefore no real transformation is occurring. Religion, new insights, and fine feelings mean little to nothing if there is no change in behavior. Jesus said, "A tree is always known by its fruit." There needs to be results, otherwise we are making Christianity unbelievable to the world. In this world, if we consider people being divided into two camps, Christian and non-christian, some who call themselves Christian are actually moving away from Christianity, and others who are not Christians, are moving towards Christianity, being led by God's secret influence.
Lewis then brings about the question concerning why there is sometimes what appears to be a bad Christian and the good Atheist. Lewis answers by saying, if Christianity is true, then any Christian will be nicer than he would be if he were not a Christian. Perhaps Christian Miss Bate's sharp tongue is obvious to all who know her, yet unbelieving Dick seems to be a fine upstanding gentleman, but in reality, these issues are a result of natural causes, such as, their upbringing and temperaments. What both these people need is new management, meaning the Holy Spirit actively working in their lives. Perhaps, by the end of Miss Bate's life, she will have learned to better control her tongue, and all of Dick's traits are not really his, but gifts from God, and perhaps poor indigestion might cause Dick to not be so nice. God is watching, waiting and working to see if they will turn to Him, and allow Him to fulfill the purpose for which they were created.
Of course, a nasty nature is deplorable in God's eyes, and a nice nature is a good thing. To convert rebellious wills costs Christ His crucifixion. As nature is in a constant state of passing away, so our earthly bodies will all pass away, yet we all have the chance to become an eternal spirit with a resurrected body if we choose to do so through Christ who came to call sinners to repentance. Many awful and nasty people seem to come to Christ in greater numbers than the so called, "nice people," because they are the ones who need help the most. Those who are wealthy and seem to be gifted by God with all that they need, with popularity and health, may in the end, see no need for God as they fail to see that all things really belong to Him, and these gifts are only borrowed and temporary. Those poor in spirit, the lonely, lowly, timid, warped, thin-blooded, sensual and even nasty ones, they are the lost sheep Christ came to find.
The Devil, as an archangel, has powers far above yours, and look what happened to him when he fell with all his demons. If you are merely content to be nice, you are nevertheless still a rebel in the eyes of God. If you are a poor creature nagged by inferiority, do not despair! Keep on! For the last shall be first and the first shall be last. Of course, niceness is something we should all pursue in our educational, medical, economic and political systems to produce a better world, but the far bigger concern is the salvation of souls. God became man to turn His creatures into sons, and give them wings to soar through the heavens so to speak. So don't miss the flight!
You will always be able to find unsatisfactory Christians, if that is what you are looking for, and may think, if that's your new man, then give me the old one, but if you are honest in your pursuit of the truth, you will see that this is evading the issue, and may eventually see on other grounds that Christianity is probable. If there is a God, in the end, we are all alone with Him, ready or not, to give an account, and to stand in His presence, is either the greatest blessing of all, or simply unavoidable and for some, terrifying.