1 Corinthians 3:2 - Matthew Poole's English Annotations on the Holy Bible

Milk signifies what the apostle to the Hebrews calls the first principles of the oracles of God, and so is opposed to sublime spiritual doctrines, here set out under the notion of meat; called strong meat, Hebrews 5:14, fit for those of full age: as young children's stomachs will not endure strong meat, so neither are sublime spiritual mysteries fit for new converts, until they have senses exercised to discern good and evil; and therefore the apostle gives this as a reason, why he had not communicated the deep things of God to them, because as yet they had not been able to bear the notion of them, nor indeed were they yet able: it should seem that there were many in the church of Corinth, who though they were true Christians, yet were not grown and judicious Christians, but had great imperfections, as indeed it will further appear in this Epistle.