Undiluted wine
Is it sinful to drink undiluted wine? A Baptist is telling me that it is sinful to drink wine that is not diluted with at least three parts water to one part wine. He claims all wine was diluted in biblical times. This seems strange since Isaiah 1:22 seems to speak of diluted wine in a negative sense.
Wine was often diluted in biblical times because water was not abundant in some places or of the best quality.
While Christians today can certainly follow an Old Testament custom or practice if they like, it is not sinful if they do not. There are no New Testament ceremonial laws. It is not sinful to drink undiluted wine.
You are correct in noting the negative sense of diluted wine in Isaiah 1:22. That verse uses figurative language to describe the spiritual unfaithfulness of the people of Jerusalem in Isaiah’s day.