Recently, a letter writer said that "Sin is, and always will be sin" and "God is never-changing." With respect to the latter, he correctly expresses a longstanding Christian view created by St. Augustine in the 4th Century and dominant until well beyond the Reformation. Even now the line "O Thou who changest not, abide with me" is sung in churches.
The idea, however, is not biblical but Greek. It was Parmenides who said, "What is, is; what is not, is not," indicating that divine or true reality is whatever does not change, an idea picked up by Plato and Aristotle. Its fateful connection to Christianity was made when Augustine, deeply influenced by Neo-Platonism, commented on the Bible's story of the Burning Bush. When Moses asked the name of the God who spoke from the bush, the reply was "I am that I am," which Augustine interpreted as "He who is." It was an exceedingly flimsy biblical justification for the Greek idea that God does not change.