Rashi on Genesis 17:1/רש"י על בראשית י״ז:א׳

1אני אל שדי I AM GOD ALMIGHTY — I am He whose (ש) Godship suffices (די) for every creature (Genesis Rabbah 46:3), therefore, ‘‘walk before Me”, and I will be your God and Protector. Similarly, wherever in the Scriptures this Divine Name שדי occurs, it signifies the idea of His sufficiency — but it all depends upon the context as to what the “sufficiency” refers to.

2התהלך לפני WALK BEFORE ME — as the Targum takes it: worship before Me — cleave to My service.

3והיה תמים AND BE PERFECT — This, too, is a command following upon the previous command: be whole-hearted in all the trials I impose upon you. According to the Midrash (Genesis Rabbah 46:4), however, it means: walk before Me by observing the precept of circumcision and through this you will become perfect, for so long as you are uncircumcised I regard you as having a blemish. Another explanation of היה תמים BE THOU PERFECT — at present you lack the power of controlling morally five organs, viz., two eyes, two ears and the membrum. Therefore will I add a letter (i. e. the letter 'ה the numerical value of which is five) to your name (אברם which equals 243) so that the total of the letters of your name (אברהם) will become 248, corresponding to the number of limbs of your body (cf. Nedarim 32b).