The similarities are in the progression in what it teaches.
The differences are caused by man.
That is as simple as I can put it.
If we were to 'sieve' through the Books, you will 'find' an 'evolution' for the "be-ing" (and I guess putting the 'being' in human beings)
The nature of prophet Daud (David) pbuh apart from his actions, wisdom, he also sang songs of praise, exalting, and stuff akin to tasbih, done so melodious, everyone knows to Whom Daud pbuh was thankful. Zabur (Psalms) = Tauhid.
Taurat (Torah). Human mechanics. Do unto others what we you want others to do unto you.. over time it became reason for revenge. Eye for an eye..
The Injil (Bible) drew attention to a higher state of being. To forgive. I can imagine it being an alien concept then.
With knowing Tauhid and believing in the messages, willingness to forgive, the final message can be delivered.
The Quran and its laws on living in a community. For humanity, lesson complete.
But the reality is that Syaitaan (Satan) solemn oath has caused much distortion and made each Book separate religions.
eace: