Why do we exist? What is the purpose of our lives? Everyone one of us asks this question at some point in our lives. Philosophers have proposed that the purpose of human existence is to be happy or to live the good life. However, the question then becomes: What is happiness and what is the good life?  Attempts to define happiness or the good life—hedonism, wealth, honor, wisdom, and virtue, to name a few—have not been satisfactory. 
As Christians, we evidently think that having the correct belief system is the most important aspect of our lives because that is how we are saved. While knowing what is true is a critical part of our existence, is the ultimate goal of our lives to believe in certain ideas? God made us finite. He made it difficult for us to know what is true. Why would he structure our existence in this fashion if knowing what is true is the ultimate purpose of our life?
Others propose that living a life according to God’s principles is the purpose of our lives. Another way to name this purpose is virtue which is defined as “moral excellence, goodness, righteousness”. While being virtuous is required by God, it is not the reason for our existence. We do good or evil because through those actions we will obtain a particular result that we desire. We do not do good or evil for their own sake.
As Christians, the reason for our existence is not to be happy; it is not to live the good life; it is not our belief system; it is not being virtuous. These are just means to an end, not the end itself. The purpose of our lives is to become like God. As C. S. Lewis states:
What [God] cares about is that we should be creatures of a certain kind or quality—the kind of creatures He intended us to be—creatures related to Himself in a certain way. 
And God can and will use everything else in our lives to accomplish that purpose.
 John Mark Reynolds, When Athens Met Jerusalem, Downers Grove, IL: InterVarsity Press, 2009, p. 210.
 Reynolds, pp. 204-218.
 C. S. Lewis, Mere Christianity, (New York: The MacMillian Company, 1952), p. 113.