Some expositors suggest that this phrase refers to another non-inspired epistle which Paul wrote to the Ephesians, a document which has been lost. I am inclined to the conclusion in the previous paragraph, however. By which, when you read, you may understand my knowledge in the mystery of Christ. This is the only way a believer can become occupied with Christ, be spiritually self-sustaining, and can have the productivity associated with Christian maturity. The Holy Spirit indwells a Christian for the purpose of teaching, among other things.
Nothing is more pitiable that to see a grownup with the mind of a child.
Yet we understand that many people are challenged thus through no fault of their own. For when for the time you ought to be teachers, you have need that one teach you again the first principles of the oracles of God, and are become such as have need of milk, and not of solid food. But solid food belongs to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil. And the things that you have heard from me among many witnesses, the same commit to faithful men, who shall be able to teach others also. Timothy was in Paul's traveling seminary for several years, and he was now a pastor himself, but a man who still needs instruction.