We all know that there are certain things that we should do, and certain things that we should not do. It is built into who we are like an engine in a car. This ability to know right from wrong is what keeps everyone from destroying each other and the planet on which we live. Yet how many of us have ever stopped to ask, "Where does this sense of right and wrong come from?" The truth is that objective morals cannot exist without a transcendent God.
Morals, without God, are merely subjective (based on the opinion of myself or someone else) or illusory (an illusion, or not real). I believe that David realized this when he penned Psalm 14. In this psalm, we see the moral argument for God's existence coming from the pages of Scripture itself.
There are a few things we should note about this piece of Scripture. First, the word "fool" here means not only someone who is foolish, but someone who is immoral, or morally deficient. It is easy for us to say that this passage refers to only atheists. However, this is not the case. Of course atheists have no objective reference for their moral values. If they want one, they must "borrow" from a worldview that believes that God exists, such as Christianity.
Second, in order for someone to "turn aside" from following God means that he or she was following God at one point. We can take this as more evidence that David was not only referring to atheists when he wrote this psalm. In the same way, in order to become corrupt, someone must have been uncorrupted prior to their corruption.
Finally, we can see that God considers it good to follow after Him. We must always remember this and, when we struggle, look to Him first. He will not leave us without guidance.
This passage also refers to anyone who does not live a moral life. It does us, as Christians, no good to look to God for objective moral values if we are not going to live like He commanded. "Love the LORD your God." "Love your neighbor as yourself." If we do not live by these values, then we are essentially living as though there is no God.
I am not preaching salvation by works. Salvation comes by grace alone through faith alone in Christ alone. However, to live in a manner inconsistent with biblical teaching will hurt us when it comes to witnessing to the those around us. David even appears to argue that, to believe in God, yet live as though He does not exist causes us to become corrupt people who do no good. This is a serious charge that we must seriously consider, as it comes from the pages of Scripture itself. We need to carefully consider what Scripture says and live our lives based on the fact of God's existence.
I have considered myself an apologist for a few years now, and there is one thing that I have learned from all of this time. This is that even the most logically sound, intelligent arguments for the existence of God will be challenged by anyone who does not want Him to exist. However, the one thing that cannot truly be challenged is a changed life. As many, if not more, people come to Christ by observing what Christ does in us than come to Him because of an intellectual argument for His existence. Remember this, and live life, following Jesus as your example.