Mormons believe in the Bible and other books of scripture

Protestants believe in something called sola scriptura, which means that the Bible is the only scripture and that it contains all of the doctrines necessary for salvation. Many Protestants do not believe that Mormons are Christians because Mormons believe in more scripture, in addition to the Bible. Mormon books of scripture include the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price. All of these books of scripture testify that Jesus is the Christ. 

There are two issues with this argument. First, the scripture that the doctrine of sola scriptura is based on comes from Revelation 22:18-19 which says “…If any man shall add unto these things, God shall add unto him the plagues that are written in this book.” However, at the time this was written, the Bible had not been compiled with the Book of Revelation as the last book in the New Testament.

Second, many other Christian denominations such as the Roman Catholic, Ethiopian Christian, or some Orthodox Christian religions believe in more books of scripture than Protestant religions do. However, Protestants still consider the members of these denominations Christian. It is a double standard to say Mormons aren’t Christians because they believe in more scripture than the Bible. So, why are Mormons not Christians, when other denominations are?