The Da’wah of the Prophet Muhammad was not restricted to a particular time or place; rather, it was a universal Da’wah; one to last until the end of time.
There is much evidence to substantiate this and some wonderful examples can be found in the Quran. Allah clarifies the timeless prophetic role of Muhammad in the following verses (which mean):
• "And We have not sent you (O Muhammad) except as a giver of glad tidings and as a warner to all mankind." [Quran 34:28]
• "Say (O Muhammad): 'O mankind! Verily, I am sent to you all as the Messenger of Allah." [Quran 7:158]
• "Blessed be He Who sent down the criterion (of right and wrong, i.e., this Quran) to His slave (Muhammad) that he may be a Warner to the 'Aalameen (mankind and Jinn)." [Quran 25:1]
There is a Hadeeth in which the Messenger of Allah said: "I have been given superiority over the other Prophets in six respects: I have been given words which are concise but comprehensive in meaning; I have been helped by terror (in the hearts of enemies); spoils have been made lawful to me; the earth has been made for me clean and a place of worship; I have been sent to all of mankind and I am the seal of the Prophets." [Muslim]
The proof that the Da’wah (call) of the Prophet was also for the Jinn is manifested in the statement of Allah, Almighty, (which means): "And (remember) when We sent towards you (O Muhammad ) Nafran (three to ten persons) of the Jinns, (quietly) listening to the Quran. When they stood in the presence thereof, they said: 'Listen in silence!' And when it was finished, they returned to their people, as warners. They said: 'O our people! Verily, we have heard a Book (this Quran) sent down after Moses, confirming what came before it, it guides to the truth and to a Straight Path (i.e., Islam). O our people, respond (with obedience) to Allah's caller, and believe in him (i.e., believe in that which Muhammad has brought from Allah and follow him). He (Allah) will forgive you of your sins, and will save you from a painful torment (i.e., Hell-fire).'" [Quran 46: 29-31]
Thus, the Prophet spread his Da’wah outside of his own people, and indeed, outside the Arab world entirely. The introduction of Islamic Da’wah spread globally after a truce was made with the Quraysh in the treaty of Hudaybiyyah. He sent messengers and wrote letters to the various kings of his time. The first of these messengers according to Ibn Al-Qayyim was ‘Amr ibn ‘Umayyah ad-Dimry, may Allah be pleased with him, who was sent to Negus, King of Abyssinia (Ethiopia). He carried a letter, rather an invitation, which stated: "Come to a word that is just between us and you, that we worship none but Allah, and that we associate no partners with Him; and that none of us shall take others as lords besides Allah. Then, if they turn away, say: "Bear witness that we are Muslims.”
Thus, the king and whoever was with him believed in the message.
The Prophet also sent ‘Abdullah ibn Huthayfah, may Allah be pleased with him, to Chosroes, ruler of Persia, with a letter that stated: "In the name of Allah, the Beneficent, the Most Merciful, from Muhammad, Messenger of Allah to Chosroes, Shah of Persia. Peace be upon those who follow the truth, who believe in Allah and His Prophet and who testify that there is no divinity but Allah and that Muhammad is His Messenger. I call you with the Call of Allah, because I am His Messenger, sent that I may give warning to him who is living, and that the Word (charge) may be justified against the disbelievers, that if they do not accept His Message, they must live with the consequence. Become Muslim and you will be safe. If you refuse, then upon you is the sin of the Magians."
When Chosroes read this, he tore the letter up, so the Prophet said: "May Allah also tear up his kingdom."
The Prophet also wrote a letter to Heraclius, the ruler of the Byzantine, in which he said: "In the name of Allah, the Beneficent, the Merciful. This letter is from Muhammad the slave of Allah and His Messenger to Heraclius, the ruler of the Byzantine. Peace be upon those who follow the right path. Furthermore, I invite you to Islam and if you become a Muslim you will be safe, and Allah will double your reward, and if you reject this invitation of Islam you will be committing a sin by misguiding your subjects. And I recite to you Allah's statement.” (which means): "Say (O Muhammad): 'O people of the Scripture: Come to a word that is just between us and you, that we worship none but Allah, and that we associate no partners with Him, and that none of us shall take others as lords besides Allah.’ Then, if they turn away, say: ‘Bear witness that we are Muslims.’" [Quran 3:64]
The Prophet similarly sent numerous messages to other kings, each conveying an identical meaning as those above.
These letters re-affirm an understood reality – that Islam is a universal religion, for all creation. Thus, it was a duty upon the Prophet to convey the pure message of Islam, calling everyone he knew, by every means available at that time with beautiful preaching in its purest form.