Heb. 5:13 And a person who is living on milk isn't very far along in the Christian life and doesn't now much about doing what is right.
In the previous verse, we are told to grow up in the faith. There were many in the church who were spiritual babies who should have been spiritual adults. These people did not understand the deeper things of faith and had to be spoon fed. Their faith should have shown signs of maturity, but they were far from it.
There are many repercussions with a child of God who never grows up. Not only does it affect his own living, it affects those people who could benefit much from a much more mature friend. Immature Christians often give Christianity, and thus Christ, a black eye. They say and do things that are not honorable before God or man. They often influence others to follow in their footsteps and fail in their faith walks.
An immature Christian has a hard time distinquishing between right and wrong. They lack clear Biblical boundaries to govern their behavior. They are not able to hear the voice of the Spirit and often stumble and fall into sin. They are easily mislead and confused as to what is or is not sin. They are very inconsistent in their walks of faith and are generally those who keep the pot stirred up. They are usually the ones who create division in the church.
God requires His children to grow up. There comes a time in which each of us needs to be weaned off the milk and start chewing on steaks. We need to let go of ourselves and let the Lord have His way in us. It is God's plan and when we don't cooperate with Him, it is disasterous for us and the church.