How can one differentiate between rejecting one of Allah's commands out of disbelief—such as someone who acknowledges the obligation of prayer but refuses to submit, or the case of Iblis—and falling short in applying the command while affirming it, and how can a person determine which of the two they are?

There is a vast difference between one who abandons an enjoined act and commits a forbidden one while acknowledging their shortcomings, and one who does so defiantly, rejecting God's حكم (ruling/judgment) and arrogantly refusing to obey it, like Iblis. Whoever rejects the Sharia out of arrogance is a disbeliever, even if they affirm that it is God's Sharia. This is one of the two types of denial (juhud). Sheikh al-Islam Ibn Taymiyyah holds that there are three categories for one who abandons prayer:

1. Denying its obligatory nature: A disbeliever by consensus.

2. Refusing to commit to performing it out of arrogance, envy, or hatred: A disbeliever by consensus, like Iblis and Abu Talib.

3. Abandoning it due to laziness and negligence, while acknowledging and committing to it: A matter of dispute.