Verse 18 continues with the discussion of infants that Benjamin broached in verse 17. The question is the standing of children and the law. The previous verse indicated that the law condemns children, but the atonement saves them with no change on their part. That concept requires an explanation.
The very first part of the explanation is the affirmation that Jehovah is just. His justice comes from the creation of the covenant and the law. When He pronounces judgment according to the covenant and the law, He is eminently just. The difference in the application of the law has to do with agency and intent. Benjamin contrasts an infant who dies in infancy with grown men. The first part of that comparison is certainly the amount of time that it lives, but it also underscores the dependency of infancy. The infant is being raised and trained, but it is dependent upon others. There are, for the very young, few agentive actions. They respond, but do not intentionally act.
Those who are grown, on the other hand, are expected to act. It is those actions that condemn them. They elect to act in ways that drink damnation to their souls. The interesting solution Benjamin suggests is that they must “become as little children.” That does not mean giving up their agency, but rather using it to return to an inculpable state. Through repentance all may return to a state of just alignment with the covenant and the law.
This ability to repent and be cleansed from sin depends upon the promise of the coming atonement of the Messiah. Note again the emphasis on the blood of the Messiah, which is an intentional connection to atoning sacrifices under the law of Moses.