The Literal Commandment
Whatever I command you, you shall be careful to do; you shall not add to nor take away from it. (Deuteronomy 12:32)
This not only says we should not change the number of commandments (neither add nor take away) but it also says we should not add or take away from each commandment. For example, it is not enough to just remember the Sabbath day... we must also keep it holy (Exodus 20:8). To do only one or the other would "take away" from the commandment.
Messiah Says
Messiah affirms the commandment "not diminish any commandments from the Law" explicitly:
Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:19)
Pictures of Messiah
How does this commandment picture Messiah? To diminish the commandments is to diminish G-d's revelation of Himself and to disregard some portion of His character. Messiah did neither and instead tells us "He who has seen Me has seen the Father." (John 14:9b)
Messiah's faithful life of obedience, upholding the commandments, reveals the Father.
How Messiah Fulfilled
Messiah obeyed the commandment to "not diminish any commandments from the Law" by not diminishing any commandments from the Law. Instead of abolishing or cancelling any of them, He instead gave them to us in their fullness.
Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. (Matthew 5:17)
Traditional Observance
The traditional observance of this commandment is expressed by both not removing any commandment (decreasing the number) as well as not diminishing any commandment (descreasing its scope or significance).
The Sefer haChinnuch indicates that anyone who attempts to disregard a command of the Lord is violating this instruction.
Other Notes
We are able to fulfill this commandment today and should not diminish any commandments from the Law.
Why? First, because G-d has so commanded it. Second, because Messiah, Himself, did not diminish them. Third, because doing so detracts from the revealed character of G-d in the world. Our job is to both obey (i.e. perform as possible) and keep (uphold as true and good) the commandments:
Let your light shine before men in such a way that they may see your good works, and glorify your Father who is in heaven. (Matthew 5:16)
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