Psalm 14:1 says: “The fool has said in his heart, ‘There is no God.’ They are corrupt, they have committed abominable deeds. There is no one who does good.”
This Psalm goes on to describe how “they have all turned aside”. Romans 3 famously quotes Psalm 14 as proof of the depravity of all mankind. But the ungodly practices of those who are corrupt and do no good have their origin in :1, where they say, “There is no God.” Their lives are corrupt because their hearts reject God.
This verse reminds us that it very much matters what you believe.
What you believe affects what you do, for your actions are merely the logical outworking of what you truly believe in your heart.
— this is true for those who outright reject God. They do not believe in Him, and they demonstrate it with their “abominable deeds”, etc. They are merely living out the logical consequences of their belief.
— this is also true of those who do claim to believe in God. One may profess faith, but the truth is, they live out what they truly believe. Our actions are an extension of what we believe in our hearts. Therefore it is not what you SAY, but what you DO, that is the best indicator of what you really believe. Your actions are the best indicator of what is in your heart.