Antithesize means to contrast or oppose something. The antonyms for this word would be words that are synonymous with agreement or similarity. Words like harmonize, agree, blend, fuse, or unite would be the opposite of antithesize. Antithesize is often used to describe two things that are opposing or contradictory, but antonyms for this word would imply that two things are in agreement or share similarities. For example, we can say that two books have similarities in plot and characters, but they don't antithesize each other. So, in the context of language, antonyms for antithesize would refer to words that unify or bring together.