This rule applies to the proper reading of any literature. For example, the heart of all Bible doctrine is the great doctrine of justification by grace, for Christ's sake, through faith. Common sense should teach us to build our understanding about justification by going to the places where the subject and all the ramifications of it are treated systematically. He is writing to a fellow minister and has no need to speak in detail. Then there is the book of James, a wonderful place where some go to build a prima facie case for justification by works.
Major heresies are often the result of turning minors into majors.
But the Mormons use this as the basis of a whole doctrine on baptism for the dead. And while we are talking about the dead, how hard it is for us humans to think rationally when we are governed sentimentally. If we want to prove something about the intermediate state badly enough, we will find a text somewhere to support it, but the chances are that we will build a great edifice on an incidental passage. Rather, we should honestly go to where the subject is treated in a systematic way. Do not interpret the systematic passage in the light of the incidental one, but the incidental must be interpreted by the systematic.