The following illustration was taken from J.P. Moreland.
Imagine I invite you over to my house one night to play a game of monopoly. On this particular night however, I explain to you that the rules are going to be a little bit different. The monopoly board is in front of us, there is a refrigerator in the kitchen, a television in the living room, some coins in front of us, and a handful of jacks. Now when it's your turn, you can do anything you want, just let me know when you are finished.
On your first turn, you line the monopoly board with hotels and think this spells the beginning of my demise. You say to me, "Okay, your turn." I take my arm and swipe it across the board, knocking all of your hotels off. I then go to the kitchen, open the refrigerator, make myself a sandwich, and say, "Okay, your turn." You say to yourself, "I'm going to try this one more time." So you take all the hotels once again and line the monopoly board and say, "Your turn." This time I turn the monopoly board upside down, flip a couple coins, watch some television, and say, "Your turn." Now if you were faced with this type of situation, it wouldn't take you long to realize that if the game as a whole has no purpose, the individual moves within the game have no value or meaning. The only way your moves within the game of monopoly have meaning is if you discover the purpose of the game and you align yourself with that purpose.
And so it is with life.
Like the game of monopoly, the only way our lives have any ultimate value or meaning is if life has purpose behind it. And the only way life can have purpose behind it is if God exists and there is life after death. Think about it. Let us suppose that there is no God and that atheism is true. What does this mean? It means that we as human beings are simply Johnny-come-lately biological accidents on an insignificant speck of dust we call earth which is hurtling through empty space in a meaningless and random universe that will eventually die a cold heat death. In reality, we are really no more significant then a swarm of mosquitoes. In a universe where there is no God and no afterlife, our actions are meaningless and serve no final end because ultimately each one of us, along with everyone we influence, will die and enter oblivion. In this kind of universe, the mention of morality is an incoherent babbling and hence there is no difference between living the life of a saint or a sociopath, no difference between a Mother Theresa and a Hitler.
IF there is no God.
But, if God does exist, and there is life after death, then suddenly the universe and our individual actions take on meaning because there is ultimate purpose to life. Our actions not only make a difference in this life but also in the life to come. Meaning, value, purpose, and reason, can only exist within a theistic worldview. The absence of these in an atheistic worldview is what philosopher William Lane Craig frequently refers to as "The absurdity of life without God."
What is the purpose of life? It is not happiness, nor pleasure, nor power, nor fame. The purpose of life is to know God and make God known. But ultimate value and meaning does not simply come from knowing the purpose, but in aligning oneself with that purpose. And this means aligning ourselves with Jesus Christ, the author and perfector of our faith. "This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent…and for this I have come into the world, to testify to the truth. Everyone who is of the truth hears my voice" (John 17:3, 18:37).