Our Sages, of blessed memory, say that the Patriarchs observed the entirety of the Torah. Regarding Abraham the Patriarch, it is said that he even fulfilled the rabbinically instituted practice of Eiruv Tavshilin (the designation of two foods prior to the festival in order to prepare for the following Shabbat). The question is asked, "Why then did he not hasten to fulfill such an important commandment as circumcision, and wait until he reached the age of ninety-nine when the Most High commanded him to be circumcised?” The answer lies in understanding the difference between the commandments the Patriarchs fulfilled before the giving of the Torah and the mitzvot we fulfill after the Torah was given. It is true that the Patriarchs fulfilled the mitzvot, but they did so at the level of their personal strength. These were mitzvot they attained because of their high capacity as Tzaddikim and holy men, but ultimately, they were men with their natural limitations. In contrast, the mitzvot after the giving of the Torah at Sinai come from God and therefore possess a divine power that transcends the limitations of creation.
STICKS LIKE TEFLIN
There are several legal consequences in Jewish practice resulting from this principle, one of which is that our mitzvot sanctify material existence, unlike the mitzvot of the Patriarchs, which remained only on the spiritual plane. For example, when a Jew takes material leather and makes tefillin from it, that leather is sanctified and becomes an element of a mitzvah; whereas when the Patriarchs fulfilled mitzvot with some material object, the sanctity did not permeate the matter of the object and therefore did not sanctify it.
It is explained in the Zohar that when Jacob the Patriarch, through the use of sticks (as recounted in Parashat Vayetzé), caused the Divine and sacred Light of the mitzvah of Tefillin to dwell in the world. That is to say, the same holiness that we generate by putting on Tefillin was generated by Jacob through the sticks. The difference lies in the fact that the Tefillin remain sacred even after they are put on, while the sticks remain mere matter, as before.
This is because Jacob the Patriarch fulfilled the Mitzvah only with his own strength and not with the force of a Divine command, and a human being does not possess the power to generate a change in earthly reality by transforming a material object into one where Divine holiness is permanently rooted.
STRENGTH FOR THE CHILDREN
On the other hand, the mitzvot of the Patriarchs possess a special virtue, for they paved the way for us and provide us with the strength to fulfill the commandments after the giving of the Torah.5 Regarding this, our Sages of blessed memory said:6 "The deeds of the Patriarchs are a sign for their children." They are not only a sign but also a legacy of strength for their children.
It is for this reason that it was required that at least one mitzvah be fulfilled by the Patriarchs not through their own strength but through the strength of Hashem and by His command. This was the mitzvah of circumcision. Therefore, Abraham did not fulfill this mitzvah on his own initiative but waited for Hashem to command it, because this mitzvah had to be fulfilled specifically through the strength of God.
A PACT IN THE FLESH
Regarding the precept of circumcision, it is said:7 "and My Covenant shall be in your flesh as an everlasting covenant." This reflects the objective of all the precepts—that Divine holiness ("My Covenant") should permeate material existence ("in your flesh") and remain there forever ("an everlasting covenant").
Through the mitzvah of circumcision, all the precepts fulfilled by the Patriarchs can become a sign for the children, and allow us, the children, to infuse Divine holiness into material existence, and thus prepare the whole world for the complete Redemption of the Messiah.
(Likutei Sichot Volume III Page 757)
NOTES: 1.Kiddushin 82,a 2.Yomah 28,b 3.See Shir HaShirim Rabbah Parsha 1 4.Part 1 162,a See Torah Or Bereshit 23:3 4*.Bereshit 30:37 5.See Torah Or beginning of Parshat Lech lecha, and Parshat Ytro 6.Or HaTorah beginning of Parshat Lech Lekha. See Ramban Bereshit 12:10 7.Our Parsha 17:13