Sforno on Exodus 6:3/ספורנו על שמות ו׳:ג׳

1וארא, in a vision, a communication by G’d just below that of the level we know as נבואה, prophecy. Genesis 18,1 was one such example of G’d “appearing” to Avraham after his circumcision.

2בא-ל שדי, by the attribute which demonstrates that I have created existence as such, as was pointed out in Genesis 17,1.

3ושמי ה' לא נודעתי להם, the letter ב in the expression בא-ל שדי applies to the word ושמי. In effect what G’d is saying is that He has not made a point of becoming familiar to the patriarchs by His attribute Hashem when appearing to them, such as in the example mentioned. This was because He never experienced the need to change the laws of nature on their behalf. Seeing that the patriarchs could not have passed on knowledge about Me which I had not revealed to them, they in turn had not been able to pass on such knowledge to their children. I have to do this now in order to ensure that I can preserve the Children of Israel as My people.