A mature man is may distinguish good from evil.

Hebrew 5:11=6:8
We have much to say about this, but it is hard to make it clear to you because you no longer try to understand. 
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 is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
The writer is frustrated in what he wants to say because of his hearers' lack of understanding. They have got stuck at the ABD level in their faith. This lack of progress is bad enough in itself. But it is symptomatic of something much more serious. They are in danger of abandoning their faith altogether. So the writer give his sternest warning.
Why?
We can not always be an infant of Christian, we need to grow up to be mature in faith.
When trial comes, those who have no faith will be thrown away and fallen away.
A mature man is may distinguish good from evil.
How?
Be strong and learn from the Bible through the Holy Spirit. Keep faith in the Lord for God is good.