An antonym for the phrase "rendering void" could be "validating," or "confirming." When we say something is "void," it means that it lacks substance, meaning or value. Therefore, the opposite of rendering something void would be to give it substance, meaning or value. For instance, if a legal decision was made that rendered a contract void, a new decision may be made that validates the contract or confirms its validity. Similarly, if a decision was made that rendered a previously accepted practice void, a new decision could be made that essentially restores its validity. In essence, an antonym for rendering void is to make something valid or meaningful.