I recall reading a quotation from an old catechism or Catholic book from the 19th century, that said the reason Jews even today are said to worship the same God as Catholics, is because the religion of the Old Testament was revealed. From what I recall reading he said, or at least seemed to imply, that believing in a revealed religion - even if they were mistaken on certain aspects of what had been revealed - is what determined if the religion believed in the true God.
I believe the quotation I'm referring to was posted here, but I can't locate it. If anyone has it, please post it again.