We are talking scripture not theology.
How can you tell?
Read the Old Testament! I am sure you don't believe that has changed too. Find in it one instance where Satan (ha-satan) is linked to, associated with, or called "the devil."
Satan is mentioned only a few times in the OT. In none of those instances is he portrayed as anything but God's loyal servant, and not a rebellious, fallen angel.
The word "devil" who is synonymous with Satan appears for the first time in Deuterocanonical books and from there in the New Testament. The fall of Satan is mentioned by +Luke, +Paul and +John. Whine this cannot be found in the mainstream Jewish canon, it is in deterocanonical books. My point was and is: the so-called "apocryphal" books were included in the Christian canon because the Apostles used them extensively, while the Protestants reject them. That makes the disconnect glaringly obvious!