Sforno on Genesis 48:16/ספורנו על בראשית מ״ח:ט״ז

1המלאך הגואל אותי מכל רע יברך, the letter ה introducing the word המלאך is not a definitive article, identifying a specific angel [seeing that we had not previously heard about him; Ed] but acts as an appeal, instruction to such angel(s) who in the past on different occasions had been Yaakov’s guardian angels(s). Yaakov appeals to his own guardian angels to bless the children if their own merit does not suffice for the guardian angels assigned to them to do the job.

2ויקרא בהם שמי ושם אבותי אברהם ויצחק. Although we know that Avraham and Yitzchok were both Yaakov’s fathers, he meant to make plain that he did not appeal to the god of Terach or Nachor who were similarly his “fathers” in the sense of being his ancestors. It is an axiom throughout Scripture that the righteous are never linked to those of their ancestors who were wicked, i.e. idolaters. The same is true in reverse as our sages say in Sanhedrin 52, (but not found there) קרינן לרשיעא אפילו בר צדיקא רשיעא בר רשיעא, “a wicked person, even if he is the son of a righteous person is still called a wicked person son of a wicked person.” This principle is invoked in order to save the righteous father the embarrassment of being associated with his son who has turned from the path of righteousness. When the ancestry of such a wicked person requires to be mentioned, one prefers to link him to another wicked person in his lineage. This is why Yaakov prayed for the lads that they should remain G’d fearing and prepared to serve Him. If they were to do this they would qualify to be described as offspring of Avraham and Yitzchok. This principle is known as יחד לבבי ליראה את ה', “let my heart be undivided in reverence for Your name.” (Psalms 86,11)