Where does the Bible itself state that the canon is closed? I don't recall that...but then I could be wrong. I agree that the canon is closed, but I wasn't aware that the Bible stated this.
To the OP-well the first answer is by faith we know the Word of God. But for someone who is trying to find their faith, they are going to need more.
A Jew and a Christian are likely to agree on the authority of the Old Testament since they are mostly identical. So when speaking to a Jew about the authority of the NT, you must make the case for Jesus indeed being the Messiah. If someone of the Jewish faith does not awknowledge this, then they are not really going to accept the NT.
I don't know anything about the Koran. It is my understanding that it was written by one man. That right there could make it suspect as being an authoritive word of God. (I may be wrong on this point, so forgive me).
The Bible on the other hand, was written by a variety of authors over centuries. Given that each book is written for its own purpose for a specific audience (not always the same) and in a specific style...and considering the overall coherency of the Bible and the historical accuracy, there is a strong case for divine inspiration.
