The Old Testament is primarily followed by Christians and Jews. Christians use both the Old and New Testaments as part of their religious scripture, while Jews primarily follow the Old Testament, which they refer to as the Tanakh.
The Jewish bible is commonly referred to as the old testament or the Tanakh or the Hebrew Bible.*The most important texts are:the Torah (first five books of the Bible)the Bible, also called the Tanakh (known to Christians as the old testament)The MishnahThe TalmudAnd various historical writings from scholars and rabbis, such as the Shulchan Aruch, the Mishneh Torah, and the Zohar.*Only the Christians refer to the Hebrew Bible as "the Old Testament".
Judaism accepts the Tanakh as its only holy book. The Tanakh is what Christians call the Old Testament, though it should be noted that no translation of the Tanakh (Hebrew Bible) can be completely accurate, due to the rich, multi-layered nature of the Tanakh.
For Jews, the Hebrew Bible is called the Tanakh. For Christians it is the Old Testament.
The term Tanakh is not analogous (although similar to) the Old Testament, which Christians do consider the Word of God. The Jewish Tanakh is based on the Masoretic Text while the Christian Old Testament is based on the Septuagint, both of which are different versions of the Old Testament books. Due to variation in meaning, Christians hold that some terms have been altered to hide the coming of Jesus. Therefore, newer versions of Tanakh are not to be referred to for scriptural support. The Old Testament, however, does has Divine States except where superseded by the New Testament.
Muslims refer to Christians as "Ahl al-Kitab" which translates to "People of the Book" in Arabic.
Islam calls their holy book the Koran. For Judaism it is the Tanakh. For Christians it is the Bible.
Basically, only when calculating the day of "Easter".
All English copies of the Hebrew Scriptures are translated from Hebrew to English. These books are always called The Hebrew Bible (or the Tanakh, תנ״ך)Christians refer to these books as "The Old Testament"
Sure, there are Christians who converted to Islam. Refer to the link below.
The majority of people who refer to themselves as "Jewish Christians" have zero Jewish heritage, they are mainly Christians.
There aren't 2 holy books in Judaism. All of the books of the Hebrew bible (called "the old testament" by Christians) are the holy books of Judaism. There are 24 books. The Torah comprises the first 5 books.