Why did Gabriel descend upon the Messenger, and how did the Messenger ascertain his identity and that what he was receiving was the word of Allah?

The prophets were believers and monotheists before their prophetic mission, and they were ready to receive revelation, so they would not confuse it with anything else. Muslims have agreed that prophets, before their prophethood, were upon pure monotheism and knowledge of Allah and His attributes, and that ignorance and doubt were absent from them. This is supported by Ibn al-Jawzi's statement in "Zad al-Masir" that the Prophet, peace and blessings be upon him, did not know the Qur'an before the revelation, and by Imam Ahmad ibn Hanbal's statement that whoever claims the Prophet, peace and blessings be upon him, was upon the religion of his people has spoken ill, and by Al-Baghawi's statement that the prophets were believers before the revelation. The fear that the Prophet, peace and blessings be upon him, experienced at the beginning of the revelation does not mean doubt; rather, it was a fear due to the intensity and power of the revelation, or because he feared that what he saw might be from Satan, as he was not accustomed to it, until complete certainty was attained after the continuous revelation and succession of verses. What further confirms this is that the Prophet, peace and blessings be upon him, saw Gabriel in his truthful dreams before the revelation began, and portents and signs appeared to him by which he distinguished the revelation from other things.