4 But the question is still further raised by what we read in the Epistle to the Hebrews: “When now for the time ye ought to be teachers, you have need again to be taught which be the first principles of the oracles of God; and have become such as have need of milk, and not of strong meat. For every one that uses milk has no experience in the word of righteousness; for he is a babe. But strong meat belongs to them that are perfect, even those who by habit have their senses exercised to distinguish good from evil.” For here we see, as if clearly defined, what he calls the strong meat of the perfect; and which is the same as that which he writes to the Corinthans, “We speak wisdom among them that are perfect.” But who it was that he wished in this passage to be understood as perfect, he proceeded to indicate in the words, “Even those who by habit have their senses exercised to distinguish good from evil.”
Those, therefore, who, through a weak and undisciplined mind, are destitute of this power, will certainly, unless enabled by what may be called the milk of faith to believe both the invisible things which they see not, and the comprehensible things which they do not yet comprehend, be easily seduced by the promise of science to vain and sacrilegious fables: so as to think both of good and evil only under corporeal forms, and to have no idea of God Himself save as some sort of body, and be able only to view evil as a substance; while there is rather a kind of falling away from the immutable Substance in the case of all mutable substances, which were made out of nothing by the immutable and supreme substance itself, which is God.
And assuredly whoever not only believes, but also through the exercised inner senses of his mind understands, and perceives, and knows this, there is no longer cause for fear that he will be seduced by those who, while accounting evil to be a substance uncreated by God, make God Himself a mutable substance, as is done by the Manicheans, or any other pests, if such there be, that fall into similar folly.
Source: Tractates on the Gospel of John (New Advent)