Hebrews 5:11-14

Concerning him we have much to say, and it is hard to explain, since you have become dull of hearing. For though by this time you ought to be teachers, you have need again for someone to teach you the elementary principles of the oracles of God, and you have come to need milk and not solid food. For everyone who partakes only of milk is not accustomed to the word of righteousness, for he is an infant. But solid food is for the mature, who because of practice have their senses trained to discern good and evil.

v 12 - though by this time - Better, although by now.  I.e., in consideration of the time they had been believers; milk - elementary truth (see 1 Cor 3:1-3 - And I, brethren, could not speak to you as to spiritual men, but as to men of flesh, as to infants in Christ. I gave you milk to drink, not solid food; for you were not yet able to receive it. Indeed, even now you are not yet able, for you are still fleshly. For since there is jealousy and strife among you, are you not fleshly, and are you not walking like mere men?; solid food - deeper truth of the Word, i.e., the teaching about Melchizedek

v 13 - immature believers do not know about or put into practice teachings of righteousness

v 14 - the mature - maturity involves (1) time (v 12); (2) growth in the knowledge of the Word of God (v 13); and (3) experience in the use of the Word in discerning between good and evil (vs 13-14)

"Hebrews 5:11-6:20 — This parenthesis blames the dullness of those to whom the letter is addressed.  As pointed out in the Introduction they were Hebrews who believed that Jesus of Nazareth was the promised Messiah, and hat, though crucified, He was living and was about to return to Jerusalem to restore the throne of David and introduce the millennial world.  They were at the same time zealous observers of the Law of Moses.  But it was possible to take this position sincerely and yet not be Christians in the sense of Romans 3:8.  Vital Christianity demanded the raising of the moral question respecting sin; the atoning character of Christ's death; the necessity of the New Birth; and of faith in Him as a Saviour from the wrath to come.  It was to all this that the Apostle would bring these Hebrews...They were urged to go on from the infancy of John the Baptist's ministry (Heb 6:1-2) and from the childhood of Pentecostal miracles (Heb 6:4-5) to full manhood ("perfection"), i.e., to real conversion.  John the Baptist preached the rudiments of the teaching respecting the Messiah — that is repentance, faith, ceremonial washings, and, in obedience to Leviticus, of laying their sinful hands on the head of the sacrificial lamb; and in calling upon his hearers to escape from the wrath to come, he preached the resurrection of the dead and eternal judgment.  The importation of what is called Christian baptism and confirmation into verse 2 is an illustration of the ignorance of verse 12." (Williams)