Allaah sent Messengers to mankind in order that people may not have any plea against Him after He has sent these Messengers. He also revealed Books as guidance, mercy, light and healing.
In the past, Messengers were especially sent to their people and entrusted with the preservation of their book. That was why their writings became forgotten and their laws became altered, because they were sent tot a specific nation on a limited period.
[ كتاب : " الإسلام - أصوله ومبادئه . للدكتور . محمد بن عبد الله السحيم - باللغة الإنجليزية ]