The picture above communicates much of the answer. A lot happens to a person when they receive Christ and are converted/born again. They know God and are known by Him. They have a new family. They have the Holy Spirit. They understand spiritual things. They desire to please God, and they have expressed faith in God. All these and more happen to the new believer.
These changes are new experiences and abilities, but they are not matured in the believer. In fact, Satan works hard immediately after the person believes in Christ to get them to go back to their former way of life. The new faith and all the internal and external changes that accompany it need to be developed in a new believer. This is called discipleship, and it is a life-long process. However, the way one develops someone who is in the Lord for years is different than when they are new to the faith.
Follow up in similar to neo-natal care. After a baby is born, they have all the abilities to sustain life, but those abilities are not matured. The baby doesn’t know to use them yet. The baby has arms and legs, muscles, a voice, and a stomach to digest food, but none of these are mature. The new believer is similar. They know God, but don’t have a mature understanding of Him. They have expressed faith, but don’t know how to express it in every area of life. They have a relationship with Jesus Christ, but don’t know how to develop it.
Follow up assumes little about what the new believer understand, other than they are a child of God and as Jesus said, “will hear my voice.” Speaking the truth simply is very important to a new believer.
The goal is to establish them in their faith by increasing their knowledge, helping them have healthy attitudes, and guiding them into godly practices.