Correction: The Old Testament is also the foundation of Christianity. It was through the Hebrews and the Jewish nation and the lineage of David that brought forth the Christ. If it were otherwise there would be not Christianity!
If you believe that Jehovah God of the Jews and Christians is identical with the Allah of Islam, you should make an indepth study of both to learn the truth of the matter.
It is Islam that “claims” Allah, the god of Islam, is the same as.
You recall wrongly. The religion got no more “respect” than did any other from of paganism, although it did give rise to the heresy of gnosticism within Christianity.