Was the Prophet's message to Heraclius, which included "Embrace Islam and you will be safe" and Allah Almighty's saying: "Say, 'O People of the Scripture, come to a word that is equitable between us and you,'" sufficient to establish the proof against him, or was the dispatched messenger verbally inviting Heraclius to Islam and the letter was merely for confirmation, and is there any evidence for this?

The words of the Prophet, may God’s prayers and peace be upon him, in his letter to Heraclius were sufficient to establish the proof against him. Heraclius believed and knew that Muhammad was the Messenger of God, but he was stingy with his kingdom and refrained from Islam. Heraclius knew of the imminent appearance of the Prophet, may God’s prayers and peace be upon him. He tried to convince his people to believe in the Prophet, but they refused. In another narration, the Grand Bishop of the Romans tried to declare his Islam, so the Romans killed him. The Prophet, may God’s prayers and peace be upon him, was given comprehensive words. His letter to Heraclius, despite its brevity, is comprehensive and beneficial. Among its eloquence is his saying: "Submit and you will be safe," for what it contains of combining meanings and conciseness.