Answer: The law (or principle or rule) of first mention is a guideline that some people use for studying Scripture. The law of first mention says that, to understand a particular word or doctrine, we must find the first place in Scripture that word or doctrine is revealed and study that passage. The reasoning is that the Bible’s first mention of a concept is the simplest and clearest presentation; doctrines are then more fully developed on that foundation. So, to fully understand an important and complex theological concept, Bible students are advised to start with its “first mention.”
Here’s an example of following the law of first mention: the first time blood is mentioned in the Bible is Genesis 4:10, when God asks the murderer Cain, “What have you done? Listen! Your brother’s blood cries out to me from the ground.” Based on this first mention of blood, the student concludes that blood equals human life. Later, we learn that God’s justice requires the blood (life) of murders (Genesis 9:6). Combining the concepts of blood and judgment, we see God executing the firstborn of Egypt but passing over the Israelites who mark their doors with the blood of a lamb (Exodus 12:1–13)—this introduces us to the idea of a substitute, an animal’s life given in exchange for a human sinner’s life. Later, God directs Moses in the building of the altar: “For the life of a creature is in the blood, and I have given it to you to make atonement for yourselves on the altar; it is the blood that makes atonement for one’s life” (Leviticus 17:11). Jumping to Isaiah 53:5, we have a prophecy of a Substitute for all sinners: “But he was pierced [i.e., He bled] for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was on him, and by his wounds we are healed.” In the New Testament, John the Baptist calls Jesus “the Lamb of God who takes away the sins of the world” (John 1:29; see also Revelation 5:6—surely a bloody lamb). Jesus was insistent that His purpose in coming to earth was to shed his blood (life) for all people (Mark 8:31–33; 10:45). The Epistles further explain: “Without the shedding of blood there is no forgiveness” (Hebrews 9:22); “The blood of Jesus . . . purifies us from all sin” (1 John 1:7).
Genesis, the book of beginnings, naturally contains many “first mentions,” including the foundations of these doctrines and concepts: divine omnipotence, creation, paradise, marriage, family, sin, sacrifice, atonement, angels, prayer, judgment, covenant, government, death, burial, etc. When asked about marriage, Jesus pointed to two “first mention” passages: “Haven’t you read . . . that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’?” (Matthew 19:4–5; cf. Genesis 1:27; 2:24).
As we can see, the law of first mention has some value in the science of hermeneutics. The rules of hermeneutics for studying Scripture may number from a few to a dozen or more, depending on the scholar or teaching institution, but the law of first mention is consistently included. Studying all the Bible has to say about a particular doctrine, including its first mention, is commendable. “Be diligent to present yourself approved to God as a workman who does not need to be ashamed, accurately handling the word of truth” (2 Timothy 2:15).
When following the law of first mention, we must be careful to also follow the other rules of hermeneutics. We cannot ignore context, for example. The fact that the first mention of a serpent in the Bible (in Genesis 2) is associated with Satan doesn’t mean that every mention of a serpent in the Bible should be interpreted as satanic (the serpent on the pole in Numbers 21:9 is a type of Christ, according to John 3:14).
One of the weaknesses of the law of first mention is the difficulty of knowing what comes “first” sometimes. Are we to look at the Scriptures strictly chronologically? According to the order of the books as we have them now? Or according to the order of the books as found in the Hebrew Bible? The law of first mention is not a hard-and-fast rule, but it can be a useful guideline for in-depth Bible study, as long as it is applied in conjunction with the other rules of solid hermeneutics.