What is meant by the Almighty's saying, "O you who have believed, believe in Allah" regarding the latter belief?

Scholars differed concerning the meaning of "those who have believed" in the verse: 1. O you who have believed, continue and persevere in your faith. 2. O you who believed in the prophets preceding Muhammad, peace be upon him. 3. An address to the hypocrites, meaning, O you who have outwardly believed, be sincere to Allah. 4. An address to the polytheists, meaning, O you who have believed in your deities, believe in Allah and His scriptures.

The first meaning is the most probable.