She sells drugs.
He sells drugs.
I am worried about the rumor that he and she/they sell drugs.
2. An appositive directly affects the subject of the sentence. The purpose is to give additional information regarding the subject. For example,
My neighbor who lives across the street shoveled the driveway for me.
"Who lives across the street" is the appositive describing my neighbor. There are no commas because I want the reader to know it was a specific neighbor out of the many I have.
Therefore your sentence, "The belief that you came late is true." It would not be set apart by commas because the phrase is necessary to know what the belief is. To set the phrase apart by commas means that it is unnecessary to the overall meaning of the sentence.