Milk of the word.

1 Corinthians 3:1-3 NASB
And I, brothers and sisters, could not speak to you as spiritual people, but only as fleshly, as to infants in Christ.
I gave you milk to drink, not solid food; for you were not yet able to consume it. But 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 ordinary people?

Paul uses spiritual milk to communicate with unspiritual people, i.e. the lost, and with new Christians who have yet to learn from their indwelt Spirit how to grasp spiritual solid food (meat).

Hebrews 5:12-13 NASB
For though by this time you ought to be teachers, you have need again for someone to teach you the elementary principles of the actual words of God, and you have come to need milk and not solid food. For everyone who partakes only of milk is unacquainted with the word of righteousness, for he is an infant.

Here the author of Hebrews refers to the initial fundamentals of the gospel as "elementary principles." Once indwelt, we must undergo Progressive Sanctification and grow more mature so that we can understand and use spiritual solid food (meat).

1 Peter 2:1 NASB
Therefore, rid yourselves of all malice and all deceit and hypocrisy and envy and all slander,
and like newborn babies, long for the unadulterated spiritual milk of the word, so that by it you may grow in respect to salvation,

Clearly our progression from our initial new born immaturity requires nourishment upon actual gospel truths (pure spiritual milk) rather than bogus corruptions which can leave some Christians as immature and less effective as Ambassadors of Christ.

 

Hebrews 6:1-2 NASB
Therefore leaving the elementary teaching about the Christ, let us press on to maturity, not laying again a foundation of repentance from dead works and of faith toward God,of instruction about baptisms and laying on of hands, and about the resurrection of the dead and eternal judgment.

Here "elementary teachings" refers to our initial or beginning information about Christ, i. e. the gospel of Christ. Thus after we are transferred into Christ because our rudimentary faith was credited by God as righteousness, we need to press forward toward Christian maturity through the process of progressive sanctification.

When we learned the essential gospel truths, we learned to turn away from works of the Law and toward simply trusting in the substitutionary sacrifice of Christ. We have learned about how we were spiritually baptized into Christ, and have committed to following all Christ commanded, including water baptism by immersion.

We understand why multiple believers ordain and purpose a believer to their service within the community of believers by the "laying on of hands." Our local churches are not made up of self declared professing believers.

And of course we know about Hades and Gehenna, and eternal punishment if our names are not written in the Lamb's book of life.

 

Lets' se if we can make some sense of "the principles of the Doctrine of Christ" you quoted from Heb. 6.

The "Oracles of God" (5:12) covers the entire Scripture including both the Old and New Testaments.

But notice that Paul is speaking of "the first" principles of the Oracles of God. He's speaking directly of the Old Testament principles (5:12) of the Doctrines of Christ.

Remember, Paul is addressing the Hebrews who had a very good understanding of the Law, but did not have the understanding of Christ in the Law. This is the main purpose of the Book of Hebrews, to explain Christ in the Law, which Paul does in amazing style. To the point that staggers the mind to be honest.

These Hebrews had been believers for a while, notice what Paul tells them.

5:12) "For when for the time you ought to be teachers, you have need that one teach you again which be the "First" Oracle of God."

Paul is telling these Hebrews by now you should have a good understanding of the principles of Christ in the Law, but you don't. Paul is being firm, as Paul is always firm when handling the Word of God.

So he tells them in 6:1, Therefore leaving the principles of the Doctrine of Christ, let us go on unto perfection." Paul is saying, let's leave behind the principles of the OT and move on to the New Testament of Christ. In other words, the New Covenant.

6:2) "Not laying again the "Foundation" of repentance from dead works."

The "foundation of repentance from dead works" is referring to the old Levitical Sacrificial system of works. Paul is telling them they don't have the knowledge they should have of this system concerning Christ, so now let's move on and I'll explain to you Jesus Christ in the New Testament/Covenant.

 

This is Paul teaching these Hebrew believers Christ in the NT, and now they can automatically place Christ in the Old Levitical System with the proper under standing of Christ in the "first principles."

Notice also that Paul said, "you have need that someone AGAIN teach you."

Someone had explained all of this to them already at conversion no doubt, maybe it was even Paul.

 

There you go! They should have already had the meat of the Word according to Paul.

But they were still using the milk of the Word.