Surah 3. Aal-Imran, Ayah 66

I.e., as to what was the true creed of Abraham. "That which is known to you" is an allusion to their knowledge of the obvious fact that many of the teachings based on the extant versions of the Torah and the Gospels conflict with the teachings of the Qur'an (Razi).

 The perception of Jesus (ﷺ).

 Abraham (ﷺ) being Jewish or Christian.

The number of sects among the Jews and Christians shows that they wrangled and disputed even about some of the matters of their own religion, of which they should have had some knowledge. But when they talk of Father Abraham, they are entirely out of court, as he lived before their peculiar systems were evolved.