Yeah the fake grace crowd screams LEGALISM is somebody tell them living in sin ain't right.
That is not what I am talking about, putting extra conditions for salvation and Christian living that aren't there in the NT is legalism or adhering to Old Covenant Jewish laws that were not a part of the NT.
For example some denominations prohibit the eating of certain foods, the only NT instructions regarding food.
Acts 15:29
"
You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things"
The food issue is not a matter effecting salvation though, sexual immorality on the other hand can, as it's repeated throughout the NT as a dangerous kind of sin that can lead to destruction, however it's not an unforgivable sin, one can repent and it's forgiven.
Another example of legalism is requiring Christians to rest on a specific day in order to be saved or prohibiting them from worshiping on a Sunday, or prohibiting them from celebrating Jesus during Easter or Christmas
Or "though must get rid of tattoos" or "men must wear suits and ties to go to church" or "though must not use certain genres of music in Christian songs."
Those are examples of legalism.
You get the idea....