What man do the Israelites believe they descended from?

Answer 1

Answer #1:

Abraham (through his grandson Jacob, who was also named Israel, in Genesis ch.35).

Answer #2:

To the above answer, add Adam and Noah (ancestors of Abraham and of mankind in general).

Answer 2

The Israelites believed they were descended from Abraham, his son Isaac, and Isaac's son Jacob.

Answer 3

The Israelites described themselves as the Children of Israel, specifically, the 12 tribes of Israel were each descended from a son (or grandson, through Joseph) of Israel, also known as Jacob. Jacob was the son of Isaac, the son of Abraham, so the Israelites consider themselves to be descended from all three patriarchs, Jacob, Isaac and Abraham. When Jews instituted a formal system of conversion, this was formulated as an adoption ritual, where the convert takes the patriarchal name "ben Avraham v' Sarah" (son of Abraham) or "bat Avraham" (daughter of Abraham). (This may also be expanded to "... Avraham v'Sarah" -- ... of Abraham and Sarah, or "... Avraham Avinu" -- ... of Abraham our Father, or even "... Afvaham Avinu v'Sarah Imanu" -- ... of Abraham our Father and Sarah our Mother." These formal forms make it clear that the convert is descended, by right of adoption, from the patriarch Abraham and not just any ole Abe.)