Paul suggests that a new convert is a child, unstable in his ways, who really does not know which end is up spiritually. He can easily be tricked and deceived.
Someone who has grown, on the other hand, is someone who is stable, who will not be swept aside by persecutions, trials, deceitful teachings, and false doctrines. He can fight these off because he knows, understands, is convicted, and continues in the truth. However, he did not get to this point without also going through a process of growth. He had to pray, study, obey, make choices, analyze, compare, look at the fruits of things, and so on. He had to set his will and change. As he does these things, growth takes place.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Nineteen)