Think about it. Why buy anything, if you're happy with what you have? Why even shop? A salesman either has to find you discontented, or make you that way, if he wants to make a sale.
Now, sometimes the discontentment is legitimate and undeniable. Your washing machine broke, you need a new one. Your roof leaks, your car keeps breaking down, your clothes are becoming too revealing. You're "discontented" with being smelly, wet, stranded, and indecent. Nobody needs to talk you into looking for something new. For that matter, our conversion to Christ springs from a God-given "discontentment" with being lost, under sin, separated from God.
But what if what you have is really okay? What does the salesman do then? He has to convince you, somehow, that it is not okay. He has to persuade you that you'd be a lot more productive with a faster computer, that you'd be a lot more attractive if you bought his line of clothes/cologne/shoes, that you deserve a better car. Then what you thought was pretty decent doesn't look so hot anymore. You're discontented, and now you're vulnerable to a good sales pitch.
It's also Satan's favorite tool. And why should Satan even imagine changing his tactics when we, gullible fools that we are, have fallen for it again and again for thousands of years?
So how can anyone counter this appeal to discontentment?
In Colossians, not only does Paul lay down solid teaching about the person and work of Christ, he also dwells on ways to make personal use of the truth. Chief among these is thankfulness. Again and again Paul either expresses gratitude, or says that all believers should be grateful, should give thanks. We see it at least in 1:3, 12; 2:7; 3:15-17; and 4:2.
Thankful people are people conscious of, and glorying in, the riches they possess. Thankful people are contented people. Contented people are immune to salesmen, whether they be peddlers of baubles and trinkets, or of false doctrine.
And so, Paul's centering on, and glorying in, the supremacy and all-sufficiency of the Lord Jesus Christ would have to flush out the false teacher. If the letter left believers rejoicing in Christ alone, grounded solidly in apostolic teaching, and uninterested in all the false teacher's supplements and additions, he was sunk. He'd have to expose himself more fully, speak more plainly. He'd have to put Christ and His work down, and put up his own additions more. He has to convince folks that what they have is not good enough.
But if God's word is everything the triune God says it is, then where is the rationale for endowing our emotions, our hunches, our intuitions, our peculiarities, with sacred and canonical status?
All we have is that Bible out there, that everyone else can see, study, learn, and meditate over just as surely as we. We have to agree with the Holy Spirit that it is what He said it was: sufficient (Deuteronomy 29:29; Psalm 119; 2 Timothy 3:15-17, etc. ad inf.), and we study it to know His mind (2 Timothy 2:7). We're on a level playing field; we have no mystical "gotcha" from God.
While itself a very liberating truth (John 8:31-32), to some it is threatening. It signals a sea-change, a paradigm-shift. It engenders panic, and panicky measures and expostulations.
But I'd point out to any and all the common factor in all of these.
Every teaching that denies Christ's divine glory begins by praising Him, and denies that it is a denial.
Every teaching that denies God's grace starts by praising it, and denies that it is a denial.
Every teaching that denies God's word starts by praising it, and denies that it is a denial.
The answer is believingly to relish what God has given us, make much of it, and just say "No thanks -- really don't need it" to supplements and substitutes.