Imagine you get a Jew, a Samaritan, a Protestant, and either a Catholic or an Orthodox round the table. You show them four books and ask "Is this scriptural?"
Genesis is accepted by all four.
Isaiah is accepted by three of them, but rejected by the Samaritan.
Acts of the Apostles is accepted only by the two Christians.
Wisdom is accepted only by the Catholic or Orthodox Christian.
And the moral of this is that it's tradition which defines scripture. The only book that comes with the label "word of God" is the Quran, and none of our four are going to accept that!