Are most Jews actually Gentiles?

There is a controversy over this question and its answer. World Jewry is divided into basically two types – Ashkenazi Jews (80% of the total) and Sephardi and other Jews (20% of the total). There are a number of sources, including Jewish sources, that claim Ashkenazi Jews are actually descendants of the Khazars that converted wholesale to Judaism around 700 A.D. and settled in eastern Europe. Since the Bible term Ashkenaz is for a descendant of Japheth and a location close to the Khazarian kingdom, there is some Biblical basis for considering this possibility (Gen 10:3; I Chron 1:6; Jer 51:27). Regardless of this controversy, it is absolute truth that the true seed of Abraham in God’s view is Jesus Christ and those that are His (Gal 3:16,28-29; 6:16).

See Also

Sermon and outline of spiritual and biological Jews: The Israel of God.
For the Bible answer identifying the seed of Abraham: Seed of Abraham.
Article about Khazars and Ashkenazi Jews: