What is the faith that the Messenger instilled in the hearts of the Companions, so they believed before the complete revelation of the Qur'an, and which is not merely an acknowledgment and submission, so that a Muslim may reach the rank of the Companions?

The faith of the Companions was a certainty in Allah and the Last Day, their glorification of Him, and complete affirmation and submission. This prepared them to be guided by the Quran and to act upon it. They acquired this faith by observing the proofs of the Prophet's truthfulness (peace be upon him) and his miracles, and by the Prophet's focus on studying matters of faith concerning the unseen and the hereafter with them, which purified their souls. It is established that they learned faith before the Quran, and their faith increased through it. An example of this is Abu Bakr (may Allah be pleased with him) believing the Prophet regarding the news of the Isra and Mi'raj, because they used to believe him in news from the heavens. The revelation of verses about Paradise and Hell first, and then about the lawful and unlawful, was one of the methods of gradually building their faith.