A checksum is a small number that is generated from a larger number which allows you to quickly check whether or not the larger number has changed. It’s a very handy tool when transferring large files on a computer, since you can instantly check whether any bits of the file have been corrupted or altered without slowly and painstakingly comparing every bit. This concept is everywhere, probably even in your pocket.
If you look at the 16-digit number on a credit card, the last digit is actually a check digit: a number generated from the 15 other digits by a simple yet clever algorithm. When you type your credit card number into a website, that algorithm tests the credit card number against the check digit, instantly revealing any typos without needing to compare that number to the entire Visa database. ISBN, VIN, and bank routing numbers all contain check digits for the same reason – they provide a really quick way to spot errors.
I’ve often wished there was an easy way to apply this concept in other areas, like knowing that pages 27 and 159 of a book will always show if the whole thing is any good, or that track 6 of a CD will instantly define the rest of the entire album. While it is never as mathematically clear as it is with credit card numbers, we can judge books, music, and entire worldviews by looking at a sample of what they produce, or the most basic and foundational ideas behind them. In scripture, we see examples of this in recognizing a plant by its fruit, or in this passage from 1 John:
Beloved, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God.
1 John 4:1-2
In this chapter, John is exhorting Christians to love one another, but to also be wary of false teachers. This is a common warning in Scripture, and the best way to recognize bad teachers and bad doctrine is to examine it carefully while holding fast to what we know is good doctrine with the knowledge and discernment that comes from accurately handing the word of truth. And while we need to test everything against the entirety of Scripture, John does give us a very simple checksum here, an extremely easy test for all spiritual ideas that will quickly show any errors.
As he explains, any doctrine that correctly defines Jesus as Christ (and all that that implies), who has come (and all that that implies), and who came physically as a man (and all that that implies), lines up with scripture and is of God. Any doctrine that contradicts this statement is anti-Christian. Matthew Henry wrote that “The sum of revealed religion is comprehended in the doctrine concerning Christ, His person and office.” This is such a simple and obvious idea that it has a mathematical purity to it: since all of God’s Word is revealing Christ, His person and office, then discerning what a teacher says about Christ, His person, and office will reveal his beliefs about the rest of scripture.
Throughout history, there have been several examples of men who professed Christ and yet denied attributes of Christ. His Godhood, His manhood, and His mission have often been subtly or blatantly opposed by men claiming to do God’s work. How confusing false doctrines can become! How gracious God is to give us verses like 1 John 4:2, 1 John 4:9, and 1 John 5:1 as touchstones or checksums to anchor us to important principles of who Christ actually is, and help us to test the teachings of men and to spot deadly errors.
Now, while mathematical checksums used in the computing world prove that a file is or is not genuine, they cannot be used to recreate the original file. Your credit card’s check digit does not contain your entire credit card number. In the same way, a single verse may be a small “sum of revealed religion,” but it is not the entirety of revealed religion. We still must know, love, and use all of Scripture, and take every thought captive to the obedience of Christ.