What is spiritual milk?
This exhortation is directed to young Christians who have only recently trusted God’s enduring Word, preached to them in the saving gospel of Christ.
Because of this miracle of regeneration just experienced, a new Christian must now “[lay] aside [the verb form here means to ‘lay aside once and for all’] all malice, and all guile, and hypocrisies” (1 Peter 2:1) and partake—as babes—of the “milk of sincerity.” The word for “sincere” means, literally, “without guile,” so he/she must now build all future progress in his/her new life—not on guile, but on guilelessness!
The phrase “of the word” is especially noteworthy. This is not the usual word for “word” (Greek, logos), but a closely related word (logikos) from which we get our words “logic” and “logical.” It is used only one other time in the New Testament, where it is rendered “reasonable” in the classic passage dealing with “your reasonable service” (Romans 12:1).
Thus, Peter is talking about partaking of a spiritual milk which is both logical and without guile. This can be nothing else (as seen in the context) than the incorruptible, eternal, regenerating Word of God, and the living Word (Jesus Christ) revealed therein.
Now, the Lord Jesus is surely logical, for He is “the truth” (John 14:6). In this same chapter, Peter also notes that Christ was without guile (1 Peter 2:22). These attributes must be equally true of His written Word. The Scriptures are not full of secret meanings which only specially trained interpreters can fathom. They are sincere, meaning precisely what they say! Neither are they naive and unscientific, but fully logical and correct in everything they say. Therefore, they are genuine spiritual nourishment for babes in Christ and will certainly enable them to “grow thereby.”
