Many sources claim that 7 is the number of perfection, and since God is perfect, 7 is His number. Let's take a look at some of the many places that the number 7 is found in the Bible.
In the book of Genesis, we read that God created the earth in 7 days. (Genesis 1) In Genesis 4:15, the Lord decrees that anyone who kills Cain will be punished sevenfold. In Genesis 7:2, Noah is commanded to take seven of every clean animal into the ark. Noah also sent out the dove in 7 day intervals. (Genesis 8:10 and 8:12) There is also the 7 years of plenty and 7 years of famine in Egypt, which Joseph interpreted from Pharoah's dream. (Genesis 41)
In the book of Exodus, the feast of Passover occurs for 7 days. (Exodus 13:3-10) The first reference to a week being 7 days is also found in Exodus, along with the pattern for the distribution and use of manna. (Exodus 16)
In Leviticus 25, the land was to be fallow during the seventh year, so the land could have a Sabbath rest. After seven 7 year cycles (7 X 7) there was then a year of Jubilee in the 50th year.
In Joshua 6, the walls of Jericho fell on the seventh day after the Israelites marched around the city seven times.
The book of Proverbs gives us two examples of the number 7. Proverbs 6:16-19 lists the 7 things that are detestable to the Lord, and Proverbs 9:1 lists the 7 pillars of the House of Wisdom.
Daniel's prophecies about the end times are built on the number 7 and multiples of 7.
There are many more references to the number 7 in the Old Testament, but now I'll move on to the New Testament, which also contains many mentions of the number 7. In Matthew 15:32-37, Jesus took 7 loaves of bread and multiplied it, and there were 7 baskets of leftover bread. In Matthew 18:22, Jesus says that we should forgive others not just 7 times, but 70 X 7.
In Acts 6:3-6, 7 men of honest report who were filled with the Holy Spirit and with wisdom were appointed to distribute the food to the widows. In Revelation, we find references to the number 7 all through the book - the 7 churches of Asia, the 7 golden lampstands, the 7 spirits of God, the scroll with 7 seals, the 7 trumpets, the 7 angels with the 7 golden bowls with the plagues, and the 7 thunders.
There are many more references to the number 7 in the Bible and in Christian and Jewish tradition that I didn't even mention. I don't want to do all your Bible research for you! LOL!
Isaac Newton discovered that there were 7 colors in the rainbow, and since God created the rainbow, wouldn't you just know it would stand as a symbol of His perfection?
Full disclosure: I adapted most of the above from the Wikipedia listing for the number 7, which you can find here. I also used BibleGateway.com for research into where the number 7 appeared in Scripture.
Thanks for reading! Until next time -
P.S.: For those of you curious about the title, I borrowed it too. The St. Louis area Mobil gas stations have a promotion where if the St. Louis Cardinals score 6 or more runs in a game, you can purchase a soft drink, a coffee, or a frozen drink for 25 cents the next day. That promotion is entitled "6 is a Serious Number," hence my "7 is a serious number" title.