Let us learn a posuk with Rashi

Chapter 28 Verse 13:
"Hashem was standing over him, and He said: I am Hashem Gd of Avraham your father and Gd of Yitzchak..."

This is the introduction to the message that Yaakov receives in his dream about the covenant between the son of Yizhaq and Hashem. The promise of the land is repeated, and Yaakov is assured that he will be protected when he leaves Canaan until he finally returns.

Hashem introduces the words of the promise with a sort of instruction. "I am Gd of Avraham..." - Yaakov understands that the voice is the voice of the family faith. However, the fathers, Avraham and Yitzchak are referred to differently. Avraham is "your father", while Yitzchak does not receive this designation.
Rashi in his discussion makes an introductory point: "We have not found in scripture, that Hashem would designate his name with the names of the righteous during their lifetime." 

It is not common to find the title 'Gd of so and so' while that person is still alive; our verse says "Gd of Yitzchak," and Yitzchak was certainly still alive. Rashi relates this to the statement in the verse in Iyyov 15:15 which says "He does not trust His holy ones". Which means that even the most righteous of men cannot be called a tzaddik until he dies. As long as he still lives we do not know how he will turn out.

This principle may be somewhat distressing: a person who has strived for certain excellence all his life is not considered righteous because he might change (somewhat?) before he dies. Still Rashi accepts the principle, and questions the presentation in the Torah in which Hashem is identified as the Gd of Yitzchak.

Rashi explains the verse in a way that maintains the principle.
"Yitzchak had become blind, 'his eyes had dimmed.' He was confined to the house and was considered as if dead. He no longer possessed an evil inclination (and it was therefore impossible that he would at this point compromise his righteousness)."

Yaakov, before taking leave of Lavan (at the end of our parsha), tells his father-in-law how he views their relationship. He points out (31:42) "...had not the Gd of my father been with me..." Yaakov explains that only the protection of Hashem had prevented 
Lavan from denying him what was rightfully his.

Yaakov further explains who the "father" he is referring to is: "Gd of Avraham and dread ("pachad") of Yitzchak".

Rashi notes: "He did not want to say 'Gd of Yitzchak' for Hashem does not associate His name with the righteous during their lifetime."

According to Rashi, Yaakov knew of this principle and adhered to it when referring to his father. Rashi adds the necessary question: When Yaakov left Canaan from Beer Sheva, Hashem introduced himself to Yaakov as "...Gd of Yitzchak" because Yitzchak had lost his sight, and was considered as though dead, and while this would have made it possible for Yaakov to refer to his father in the same way, nevertheless Yaakov was afraid of imitating Gd in this matter and invented a designation, translated as the "dread" of Yitzchak.

It is important to remember that this problem did not originate with Yitzchak, but is mentioned earlier, in reference to Hanoch. Chapter 5, verse 24 (Bereshit) we learn: "And Hanoch walked with Gd, then he was no more, for Gd had taken him".The verse does not say that he died but that Gd "had taken him."

Rashi explains: "He was a righteous person but lightheaded (not stable?). There was a chance that he would not maintain his present high standard and might regress to doing evil. So Hashem removed him from the world, and he died before his time. He was not able to complete his allotted years in this world."

Hanoch was taken from this world; he did not die a natural death, in order to insure that he would die a tzaddik. Yitzchak, however, was blind and there was no danger of his status being changed during the ensuing years of his life.

Hashem hinted at the special status of Yizhaq when he introduced himself to Yaakov in the dream of the ladder. However, even though Yaakov knew that Hashem had already determined the status of Yitzchak, he did not feel that he could imitate that introduction and say the words "Gd of Yitzchak," because the future generations reading the Torah might get the wrong message.

When we look upon a person and say that he is a tzaddik, we mean that he is a special person at this moment. The truth of righteousness is only determined by heaven after one dies. Even the tzaddik is obliged to assess his status constantly, and to insure that his position is not being compromised by any misstep that he might take.

Gut Shabbos.