13 Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. 14 But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil.
The Priestly Ministry; 5:1-10
1 Every high priest is selected from among the people and is appointed to represent the people in matters related to God, to offer gifts and sacrifices for sins. 2 He is able to deal gently with those who are ignorant and are going astray, since he himself is subject to weakness. 3 This is why he has to offer sacrifices for his own sins, as well as for the sins of the people. 4 And no one takes this honor on himself, but he receives it when called by God, just as Aaron was.
5 In the same way, Christ did not take on himself the glory of becoming a high priest. But God said to him, “You are my Son; today I have become your Father.”
6 And he says in another place, “You are a priest forever, in the order of Melchizedek.”
7 During the days of Jesus’ life on earth, he offered up prayers and petitions with fervent cries and tears to the one who could save him from death, and he was heard because of his reverent submission. 8 Son though he was, he learned obedience from what he suffered 9 and, once made perfect, he became the source of eternal salvation for all who obey him 10 and was designated by God to be high priest in the order of Melchizedek.
OT priest – chosen from the human family, to represent mankind to God. He is to have a controlled sympathy for sinners, but to deal with sins by sacrifice rather than overlook them. The honor of the high priest goes only to the one selected by God, as was Aaron.
In NT, Jesus became the high priest.
Chapter 5:11-6:20. Warning Against Falling Away (the 2nd of the 4 warnings)
11 We have much to say about this, but it is hard to make it clear to you because you no longer try to understand. 12 In fact, though by this time you ought to be teachers, you need someone to teach you the elementary truths of God’s word all over again. You need milk, not solid food! 13 Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. 14 But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil.
These people, so knowledgeable about the OT priesthood, seem unable to grasp the meaning of Christ’s priesthood. They are like unskilled infants, blindly sucking at milk when they should be mature and spiritually sensitive, having grown by constantly distinguishing between what is good (beneficial) and evil (dangerous or injurious). The warning, then, is against continuing in immaturity.