Hebrews 5:13-14 (NIV)
Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil.
It makes sense that a newly converted Christian would not know much about the deeper truths of the faith. But just as we expect infants to grow and mature, we should expect Christians to do likewise. It is not right for a Christian not to grow. Some of the more difficult concepts of the faith, like dying to self, go against our natural inclinations, perhaps even appearing to be a bad thing. With maturity comes the ability to understand the difference and whet righteousness is really all about.
So how does one mature? By digging deep in to the word of God and then by putting into practice (constant use) what is found there.