The Word of God is eternal.
The Idea of God may be eternal. But the Word is constantly changing, just as language is constantly changing. All ideas, godly or otherwise, must be interpreted by human beings and expressed in words.
Obviously, translation of an idea from one language to another will change the original meaning of the text. For this reason Muslims accept only the original Arabic transcription of the Qur’an by the Prophet Muhammad as the true text, avoiding many of the misunderstandings that plagued the Christian scriptures over the centuries due translation from Hebrew and Aramaic to Greek and again to vernacular languages.
But even within the same language, the meaning of each word will change over time. This can be due to anything from technological innovations to the synthesis of clashing cultures, from which come new philosophies, traditions, and practices.
Some religions—or rather, interpreters of religious scriptures—argue that there is a pure state of their religion that is defiled by the influx of the ideas of outside influences. For example, one religious sect of Christians might insist that followers practice the holy days as God intended, without any pagan or infidel antecedents and innovations, such as costumes on All Hallows’ Eve, or Christmas trees on Christmas. However, all religions in their past and present states were and are the result of the synthesis of merging cultures. Change and adaptability are core elements of humankind. Of all species for that matter. For if Nature had not intended there to be change and synthesis, we would not have meiosis. We would be single-celled prokaryotes spawning identical copies of ourselves via mitosis.