Hebrews 5:12

For when for the time ye ought to be teachers
These Hebrews had had great advantages; they were not only descended from Abraham, and had the law of Moses, and the writings of the Old Testament, but some of them had enjoyed the ministry of Christ, and however of his apostles; and it was now about thirty years from the day of Pentecost, in which the gifts of the Holy Ghost were bestowed in such an extraordinary manner, and a large number were converted, and a church state settled among them; and therefore considering the length of time, the opportunities and advantages they had enjoyed, it might have been expected, and indeed it is what should have been, that they would have been teachers of others, some in a private, and some in a public way: from whence it may be observed, that to have time for learning, and yet make no proficiency, is an aggravation of dulness; moreover, that men ought to be hearers, and make some good proficiency in hearing, before they are fit to be teachers of others; also, that persons are not only to hear for their own edification, but for the instruction of others, though all hearers are not designed for public teachers; for to be teachers of others, requires a considerable share of knowledge: to which may be added, that the churches of Christ are the proper seminaries of Gospel ministers. But this was so far from being the case of these Hebrews, that the apostle says of them,

ye have need that one teach on again which be the first principles
of the oracles of God;
by the oracles of God are meant the Scriptures, not the law of Moses only, but all the writings of the Old Testament, which were given by the respiration of God, and are authoritative and infallible; and by the "first principles" of them are intended, either the first promises in them, concerning the Messiah; or the institutions, rites, and ceremonies of the law, which are sometimes called (stoiceia) , elements, ( Galatians 4:3 Galatians 4:9 ) where the same word is used as here; and which were the alphabet and rudiments of the Gospel to the Jews: or else the apostle designs the plain doctrines of the Gospel, which were at first preached unto them, in which they needed to be again instructed, as they were at first; so that instead of going forward, they had rather gone back:

and are become such as have need of milk;
of the types, shadows, and figures of the law, which were suited to the infant state of the church, who by sensible objects were directed to the view of Gospel grace; or of the plain and easier parts of the Gospel, comparable to milk for their purity, sweetness, nourishing nature, and being easy of digestion:

and not of strong meat:
such as the deep things of God, the mysteries of the Gospel; those which are more hard to he understood, received, and digested; such as the doctrines of the Trinity, of God's everlasting love, of eternal election and reprobation, of the person of Christ, the abrogation of the law