Several years ago, it was popular to say, "You are what you eat." In a literal sense, it's a true statement. If all you ever eat is pizza, though you won't turn into a pizza, (I hope!), the nutrients of only what you consume will be present in your body.
Similarly, The spiritual (or otherwise), food we put into our minds becomes what we think about and who we are. In Philippians 4:8 we read, "Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable - if anything is excellent or praiseworthy- think about these things. When we fill our hearts with these things, they become a description of who we are.
But what happens if we fill our minds with ungodly, unholy, even unruly things. What you think about is what will consume your mind and your time. There is no way to continually hide what you are thinking. It will manifest itself in your actions and words. Some people are expert at pretending to be what they are not, but in the end whatever our minds take in is what we will manifest at some point of weakness.
It is better to allow the Lord to fill our minds with His holiness than to take on the ways of the world.