The carnal person is a believer who has not reached spiritual maturity. This individual is controlled by their old sinful nature. They willfully say yes to selfish inclinations and no to the will of God. Their obedience is to their natural appetites rather than to the things of God.
The Apostle Paul encouraged the believers to go on to spiritual maturity.
And I, brethren, could not speak to you as to spiritual people but as to carnal, as to babes in Christ. 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 are envy, strife, and divisions among you, are you not carnal and behaving like mere men? (1 Corinthians 3:1-3).
Carnal
Their actions are characterized by the flesh, the old sin nature. The person is controlled by their old nature and its sinful appetites. They willingly say yes to selfish inclination and no to God's desires.
Understanding Of The Bible
The Bible says that the carnal believer understands milk - not solid food. What is milk versus solid food truth? Some say it is a matter of content. Milk is the gospel message and the easier doctrines, while solid food is the harder doctrines such as prophecy. It is probably best to see it as a matter of depth, not different doctrines. The carnal believer restricts themself to the abc level and never gets deep into the Word.
Lack Of Growth In Their Life
The carnal Christian should have outgrown the baby stage long ago. They remain stagnant and unproductive. A Christian who is controlled by the old sin nature does not experience normal growth.
Operates Like Unbelievers
The carnal Christian operates like unbelievers. They behave like the natural (unsaved) person both internally and externally. Internally they will think and behave like a self-centered person. Their outward behavior will be no different than the unbeliever.
Remedy
The carnal man needs the power of the Holy Spirit to break control of the old sin nature. This is their only hope since they are acting like unbelievers.
Summary
The carnal believer is one who is not yielding to the Holy Spirit. The only way this carnal lifestyle can be changed is by allowing the Holy Spirit to take control.