The first obligation of a Muslim is to believe in and affirm the Shahada (the declaration of faith). The Shahada states:
The Shahada binds Muslims to follow the Commands of Allah and the sayings of Prophet Mohammed (pbuh)“Ashhadu an la ilaha illallah wa ashhadu anna Muhammadan rasulullah.” (I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah.)
Punishment
The ultimate consequence for not accepting the Shahada (the declaration of faith) is eternal punishment in the Hereafter, specifically in Hell (Jahannam), for those who knowingly reject the truth of Islam after it has been clearly conveyed to them.
Reward
The reward for accepting the Shahada is forgiveness, purpose, and belonging in this life and eternal Paradise and Allah’s mercy in the Hereafter, provided the individual lives according to the Commands of Allah and the sayings of Prophet Mohammed (pbuh)