What was the allusion behind the water and wine image?
The water and wine image alludes to corrupting or diluting something pure, as when wine is watered down and becomes worthless. It represents false teachers corrupting believers, which Paul directly addresses in Galatians, strongly contrasting with his own uncorrupted spiritual commission.
Les teaches that Isaiah 1:22 uses simple language about water and wine, something the Jews understood as part of everyday experience. He asks what water does to wine and illustrates that watering down an expensive French wine makes it something not even worth trying to swallow. The allusion, Les explains, is that somebody was corrupting them, and that is exactly what Paul is talking about back in Galatians. Paul did not do this with his spiritual commission. So the water and wine image alludes to corrupting or diluting the pure message, making it worthless.
Thy silver is become dross, thy wine mixed with water: