Genesis 17:1

And Abram was the son of ninety and nine years, and the Lord appeared to Abram, and said to him, I am El Shadai; serve before Me and be perfect (shelim) in thy flesh.

‘E-IL SHA-DAI.’ These are two distinct Divine names, each one descriptive in itself. Now the meaning of the word eil is “mighty”, derived from the expression, Eilei Moab (the mighty ones of Moab). The meaning of Sha—dai, according to Rashi, is “He whose G-dship suffices for every creature.” In the book Moreh Nebuchim the Rabbi, Note 139. explained that the name Sha—dai signifies “he who is sufficient.” That is to say, He does not require the existence of what He created or the conservation of any other being; rather, His existence is self-sufficient. Rabbi Abraham ibn Ezra explained in the name of the Nagid that the name is from the root shodeid, meaning “victor and prevailer over the hosts of heaven.” This is the correct interpretation, for the name Sha—dai represents the attribute of power which conducts the world, concerning which the Sages have said that it is “the attribute of Justice of the world below.”
The reason for mentioning this Divine Name now is that with it are done the hidden miracles for the righteous, to deliver their soul from death, and to keep them alive in famine, as well as to redeem them in war from the power of the sword, just as all the miracles done to Abraham and to the other patriarchs, and as all the blessings and curses mentioned in the Torah in the section Im Bechukothai and in the section Vehaya Ki Thavo. These blessings and curses are all miracles for it is not in nature that the rains should come in their due season when we worship G-d, nor are the skies to be like iron, Verse 19. if we plant our fields in the seventh year, and similarly all promises in the Torah. Rather, they are all miracles by which the disposition of natural law is overpowered, except that no change from the natural order of the world is noticeable, as was the case with the miracles done through Mosheh Rabbeinu with the ten plagues, the dividing of the Sea, the Manna, the well, and their like, for these are miracles which openly changed nature, and they were done with the Tetragrammaton which He told to him. It is for this reason that He now told Abraham our father that He is the Almighty, the Victor Who will prevail over his constellation of birth so that he will have a son, and thus there will be a covenant between Him and his seed forever, meaning that the portion of the Eternal is His people, and that He will lead them at His own will, as they will not be under the …

וירא ה, a מראה is always a vision one level lower than true prophecy. This is why G'd said to Moses in Exodus 6,3 ושמי ה' לא נודעתי להם, "but I had not revealed to them My true name Hashem."

אני א-ל שדי, "I am the One Who is sufficient unto Himself by My very existence to carry out My special function the creation of the universe all by Myself without assistance by anyone else. [The word די expresses this self-sufficiency of the Lord/Creator. Ed.] The very opposite is true of all created phenomena other than the Creator. [the difference between G'd and creatures is that G'd is a "self-starter," whereas all other phenomena require something to activate them even when performing the functions for which they have been created. Ed.] This all means that the recipient of a visual image, מראה, a vision, becomes aware of the existence of the Creator and His power, without him however becoming aware of underlying causes of such. The patriarchs became witnesses to G'd performing "invisible" miracles, as distinct from the "visible" miracles performed by Moses and other prophets after him to some extent. [an "invisible" miracle maybe recognized as such because the time and place has been foretold to the beneficiary, as in the case where the angels told Avram that Sarah would give birth to a son within the year. "Visible" miracles, are for instance, water turning into blood, or all the firstborn of one nation dying at the same time [after this event having been announced.] the latter type of miracle says more about G'd as the originator of all that occurs in the universe than does the former kind of miracle. This latter kind of insight was not revealed by G'd before Moses, as we quoted form Exodus 6,3.

התהלך, an instruction to conduct himself in this manner in all his activities.

לפני, as if you were looking for guidance Me in all your undertakings and trying to emulate Me. We have adopted this concept when we inscribe our synagogues with the verse שויתי ה' לנגדי תמיד, "I am ever mindful of the Lord's presence." (Psalms 16,8)

ויהי אברם בן תשעים ותשע שנים, after 13 years had passed since Ishmael was born G'd commanded Avram about the circumcision. G'd had waited with this command until Avram would be close to 100 years old in order to make the miracle of a 100 year old man and a ninety year old woman having a child all the greater. Also, Avram, subjecting himself to the operation involved in circumcising himself would acquire an even greater merit if he did so at such an age when his physical virility had already declined. G'd issued the command before Yitzchok would be born in order that Yitzchok would be the product of a union with a Jewish father, i.e. a father who had been circumcised at the time when he impregnated his wife with his sperm. Seeing that the circumcision weakens the reproductive organ of the person undergoing it, especially at such an age. Avram's producing a son after that experience was therefore all the more remarkable.

אני א-ל שדי, the word שדי means "strong, victorious." (root שדד) We find the term appearing in that sense in Ezekiel 1,24, כקול מים רבים כקול שדי, "like the sound of mighty waters, like the sound of Shaddai." Also in Ezekiel 10,5 כקול א-ל שדי it appears in the same sense. G-d meant to tell him that although you are physically weak and Sarai is barren, I, G'd will overcome all these impediments and the laws of nature are subject to My control, so that I can change them whenever it suits me. G'd referred to this power again in Exodus 6,3 when Moses had despaired of hismission. He told them that he had already demonstrated ths power when He made the three matriarchs, all of whom had been biologically barren, overcome such a handicap so that each bore a child or children. G'd had responded to the prayers of these people or their husbands and altered the laws of nature in order to fulfill their requests. When the word שדי is meant to be an attribute, it should be spelled שדדי, shaddeday." It would then mean something akin to Proverbs 28,23 מוכיח אדם אחרי חן ימצא, i.e. that as a result of being rebuked, the subject of the rebuke will eventually find favour (in the eyes of the rebuker).

התהלך לפני והיה תמים, “walk before Me and become complete, free from blemish.” In order for Avraham to achieve this stature, G–d added the letter ה, a metaphor for His name, to Avraham’s name. The numerical value of the letters in Avraham’s name would then amount to 248, the number of limbs in his body. This is the conclusion arrived at in the Talmud, tractate Nedarim folio 32. According to Romi bar Abba, the reason why Abraham was first known as אברם and subsequently as אברהם is that originally G–d gave him control over 243 limbs, whereas from now on he had control over all of his 248 limbs. They are: two eyes, two ears, his torso, his head. If you want to realise how important the physical act of circumcision was, you will find this explained on folio 31 of tractate Nedarim. In the commentary known as Mechilta, it is stated that the reason why both Avraham and Sarah are never again referred to by their original names, as opposed to Yaakov, who after G–d changed his name to Yisrael is frequently still referred to by his original name Yaakov, this is because after both had their names changed, their original names had not been obliterated completely as the letters of their original names had been retained also as part of their new names. Their names had only been “improved.” Israel being a totally new name was not meant to invalidate his original name. The only reason we know this is because the Torah continues to make use of it. [The name Yaakov, given to him by outsiders, not his father, had suggested that he was somehow inferior spiritually already from birth. The angel in his nocturnal wrestling match had alluded to this. (Genesis 32,29) The Torah by continuing to use it, shows that in G–d’s eyes it never reflected upon him in a negative fashion. Ed.] The sages of the Mechilta add further that the reason why the name of the third patriarch, Yitzchok, has never been changed or amended, is that he had been named by G–d Himself, long before he had even been born. (Verse 19 in our chapter)