sit in judgment on (one)
To make a judgment about one (for something they have done). The defendant's notoriety made it difficult to find anyone who could sit in judgment on him without bias.
sit in judgment (up)on someone or something
to make a judgment about someone or something. (Upon is formal and less commonly used than on.) I don't want to sit in judgment upon you or anyone else, but I do have some suggestions. There is no need to sit in judgment on the proposal at this time.