To call Abraham a Jew is a bit anachronistic. Abraham is indeed the first patriarch of the Jewish people, but the term Jew as a reference to any member of the people now called Jewish was first used in the Book of Esther in The Bible. Later in Jewish history, there was a tendency to project Jewish practice back on the patriarchs, so you could say that after the Babylonian Exile, the Jewish community as a whole started considering Abraham to be a Jew. Before the exile, tribal identities were more important, and the collective terms "am Israel" (people of Israel) or "benei Israel" (children of Israel) were used to refer collectively to the 12 tribes. Recall that Israel was the name assumed by Abraham's son Jacob after the incident at the Ford of Jabbok.