أحسن الله إليك YaSheikh, If I believe something to be obligatory when there is a difference of opinion in it, exmpl: Niqab, is it correct to believe that anyone who dosent wear it is sinful, even if they believe its mustahabb? Or are they NOT sinful if they take the opinion its mustahabb?

Ustadh AbdulAziz al-Haqqan

When you believe it is obligatory, then the one who doesn't do it is a sinner by doing so. However, do not criticize them or be harsh with them. Instead, you can have a nice Academic discussion if you want to advise them and show what the evidence is.