Three rules for reading Scripture correctly: read verses in context so they don't contradict surrounding passages, check the original-language meanings of key words, and find where a concept first appears in the Bible to understand God's original design for it.
We have to understand that if we are going to properly interpret the Bible, we must apply the law of context, which means I have to know what the verses are saying before the text, after the text, and my understanding of the text can't contradict either one of those or any other passage in Scripture. That's the law of context. The law of definitions: I have to look up the definition of every word in a passage in its original language, or I do not understand the text. If I don't know what the words mean, I can't know what the word means. And then lastly, the one you talked about, the law of first mention. Because God does not change, he's the same yesterday, today, and forever. However he mentions anything in scripture, that's his original design for that thing.
