The phrase "didst one part" does not have a direct antonym as it is not a commonly used phrase in modern English. However, if we break it down into its individual parts, "didst" can be considered a variation of "did" which means to perform an action in the past. Therefore, the antonym for that would be "didn't" or "did not". Similarly, "one" can be antonymed as "many" or "multiple". Lastly, "part" can be considered the opposite of "join" or "unite". Thus, the antonyms for "didst one part" could be "didn't unite many" or "did not join multiple".