In Colossians, Paul is dealing with false teachers. Focusing on the big picture, what we see is that these false teachers were adding something to the gospel. They required the Colossians to do more than believe in Jesus in order to be saved.
Here is the thing: when we add something to the gospel, we are really subtracting from it. Christ plus something = nothing. In other words, Christ plus something = no salvation.
Why does Christ plus something = nothing?
The salvation God provides through Jesus is by grace alone. No works are required. Paul says in Ephesians 2:8-9:
"For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.”
Grace by definition excludes works, so much so that if you add one work to grace then it’s no longer grace that you are saved by but works. Paul makes this clear in Romans 11:6 when he says, "But if it is by grace, it is no longer on the basis of works; otherwise grace would no longer be grace.” It is crucial we get Paul’s point because our eternal destiny hangs on it. If you add just one work to grace, it means you aren’t being saved by grace, but by your works. Our own works can't provide us with salvation (Gal. 2:16). Only the work of Jesus can provide salvation. Only the pure gospel saves.
You can think about it like this: If you were to add one drop of poison to a glass of pure water, you could no longer say that that was a glass of pure water. Instead, you would have to say it is a glass of poisoned water—one that might look like it would save you from thirst and dehydration, but in reality, would kill you.
It is the same way with grace. When we add just one human work to salvation, the gospel of grace becomes poisoned so that when you drink of it, it no longer saves but kills. Sure, it might look like it provides salvation just like the glass of poisoned water looks like it will refresh, but it won’t. That’s because Jesus plus something = nothing, but Jesus plus nothing = everything.
I know the idea that we are saved by grace alone baffles us at times, and I know it is hard for us to get past the idea that we don't have to perform any works to be saved. Paul, however, couldn't be clearer. Jesus' work alone is sufficient for life and godliness. I know it's hard to believe, but it's true. Jesus is enough!