What does it mean not to obey an ungrateful sinner or a disbeliever?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

What is meant by this prohibition is the prohibition of obeying the one who is indulged in sin or disbelief. Some of the scholars interpreted the ‘sinner’ to mean the hypocrite, while some others interpreted it to mean a person who is immersed in desires; Al-Qurtubi said, “{And do not obey from among them a sinner} meaning a sinful person, {or ungrateful [disbeliever]}, meaning the disbelievers.”

Ibn Katheer said, “{And do not obey from among them a sinner or ungrateful [disbeliever]}, meaning, 'do not obey the disbelievers and the hypocrites if they wish to deter you from what has been revealed to you. Rather convey that which has been revealed to you from your Lord and put your trust in Allaah, for verily, Allaah will protect you from the people.' The sinner is the sinner in his deeds and the disbeliever is the disbeliever in his heart.”

As-Sa'di said, “Do not obey the polytheists who are immersed in desires or who go to extremes in disbelief and misguidance.”

Allaah knows best.