Does the condition "that the act be non-existent" contradict His, the Almighty's, saying: (O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger)?

The noble verse does not contradict the principle that a mandated action must not already have been performed, for several reasons:

1. The People of the Book's Faith in Muhammad (peace be upon him): The required faith is that the People of the Book believe in Muhammad (peace be upon him), which they did not possess. They believed in their prophets and their scriptures, but they disbelieved in Muhammad (peace be upon him) and the Qur'an. Thus, the verse calls them to believe in him and in what was revealed to him.

2. Steadfastness in Faith and its Increase: What is required of believers is to remain steadfast in their faith, and to increase and perfect it, for faith waxes and wanes. Therefore, seeking an increase in faith is seeking something incomplete, not an acquisition of what is already possessed.

3. Address to the Hypocrites or the People of the Book: The address might be to the hypocrites (O you who have believed with your tongues, believe with your hearts) or to the People of the Book (O you who have believed in Moses and Jesus, believe in Muhammad).

All these points refute the specious argument that the verse demands the acquisition of what is already possessed.