Benjamin appears to be responding to an undercurrent in the religious controversy that had split his people. Benjamin is teaching a concept about children here, and doing so in a way that presumes that there was some controversy on the subject. To understand the topics that Benjamin is addressing we need to examine the way in which he expounds this doctrine.
First, Benjamin begins his argument with “if it were possible that little children could sin.” This very beginning presumes that the people understand that little children are not capable of sin. Benjamin is not explaining this principle, but simply using it as part of his example. Notice also that he presumes that the people already understand whatever demarcation there is between the “children/not sin” and “not children/sin” age.
His next significant comment is that the blood of Christ atones for children (he says that it atones for their sins, meaning those things that they have done that would be sin were they capable of sin). Other than simply stating that Christ atones for children, why is Benjamin bringing this up?
It is critical that we remember that this is the first argument after he has declared that Christ is provides the atonement even for the law of Moses. The context Benjamin has set up is one of contrasting the law of Moses to the future Messiah. Right after declaring that Christ is the reason that atonement may be achieved under the law of Moses, he turns to children. Benjamin is carefully crafting his speech, not moving randomly from one topic to another. In this case, the second contrast between the law of Moses and the hope in Christ has to do with children, and presumes an argument not made explicit.
In Nephi’s teachings on baptism the emphasis is on the choices that one makes to enter in to the baptismal covenant:
“2 Ne. 9:23 And he commandeth all men that they must repent, and be baptized in his name, having perfect faith in the Holy One of Israel, or they cannot be saved in the kingdom of God. 24 And if they will not repent and believe in his name, and be baptized in his name, and endure to the end, they must be damned; for the Lord God, the Holy One of Israel, has spoken it.”
“2 Nephi 31:5 And now, if the Lamb of God, he being holy, should have need to be baptized by water, to fulfil all righteousness, O then, how much more need have we, being unholy, to be baptized, yea, even by water!”
“2 Nephi 31:10 And he said unto the children of men: Follow thou me. Wherefore, my beloved brethren, can we follow Jesus save we shall be willing to keep the commandments of the Father?”
In each of these cases, the baptism and gospel covenant require cognizant action. Baptism requires prior repentance, which can happen only upon cognizance of the need to repent. Jesus was baptized as a man, and therefore we follow his example. When we follow Jesus, we must keep commandments, presuming an ability to make the choices needed to follow. In each of these cases, baptism requires the ability to make responsible choices. None of this would appear different to modern LDS, but what of the context of Benjamin’s people?
If we remember that Benjamin’s people followed the law of Moses, we must remember that for those under the law, salvation came through the covenant. The covenant was part of the birthright, and signaled physically in the male by circumcision after 8 days. Thus in the law of Moses, salvation in the form of the covenant begins with infants, yet with the redemption of Christ it forcibly comes later in life. For Benjamin’s people, the dissonance between the law of Moses and the hope in Christ concerning children was apparently a point of discussion, even of controversy.
What Benjamin is doing with the mention of children is to bring the atonement of Christ into an arena that the dissenters would have claimed as the exclusive domain of the law of Moses -–the salvation of children. Benjamin declares that the atonement has the power to save those who are not yet under sin. While the atonement saves us from sin, it also saves us before sin. Just as the blood of Christ is the effective aspect of Mosaic atonement, so too is the blood of Christ the effective aspect of the salvation of children - not the blood of the circumcision.
The rest of Benjamin’s argument is important for understanding verse 19 below. Having noted that children are not capable of sin, he notes that they are still “fallen.” Specifically, the language is: “as in Adam, or by nature, they fall.” The critical piece of this information is the association of Adam and the fall (which we expect) and the concept of “by nature” which is unique to Benjamin. For Benjamin, “nature” is equated with the Fall. This fall has occurred in children who cannot sin, so the “fall/nature” happens because of the fall of Adam, not a personal defect in the child. This definition is critical to understanding Benjamin’s “natural man” in verse 19.