I fed you with milk and not with solid food; for until now you were not able to receive it, and even now you are still not able;
for you are still carnal. For where there are envy, strife, and divisions among you, are you not carnal and behaving like mere men?"
1 Corinthians 3:1-3 (NKJV)
A while ago, when I was in the USA, I was out for a walk with my mom. As we walked, we saw a small bird nearby that was hoping around from place to place. My mom made the comment, "Why hop when you can fly?". While the bird had the ability to fly it was not doing so; it may have been injured, although it did not seem to be, or there may have been some other reason. But whatever the reason was, the bird was just hopping and not flying, it was not doing something it should have been able to do.
When Paul wrote to the Corinthians, he said that they were carnal people and not spiritual. They were born again, believers in Jesus, but their lives were dominated by natural things and not spiritual things. Paul said that he could not speak to them the way he wanted to because they were so focused on the natural side of life and not spiritual truth. Just like us, the Corinthians needed to live a life focused on everything that God has provided through Jesus and not natural circumstances.
Paul said that the Christians in Corinth were acting like "mere men"; they were acting like ordinary people and not like spiritual people. In other words, they were walking when they should have been flying. Just like the bird that had the capability to fly but was just hopping around, Christians, because of Jesus, have the possibility to live a life above the things in the world. If we try to live the life God has called us to while focusing on natural truth and not spiritual truth, we will never achieve what God has planned. Why be carnal when you can be spiritual, why walk when you can fly?