Assalamu Alaykum,
I think the way you write the Question is wrong. Islam has always been on this world. For Allah sent many Prophets to mankind to teach them Tawheed. Worshipping Allah alone without associating any partners to him.
Perhaps you meant, what it means if we were not Muslim.
Obviously, if someone was not Muslim then they would be living their life in false meaning and without knowing Allah, the One Creator of all the World and universe.
Allah tells us in the Quran with his Ayaat (Signs):
And strive hard in Allâh's Cause as you ought to strive (with sincerity and with all your efforts that His Name should be superior). He has chosen you (to convey His Message of Islâmic Monotheism to mankind by inviting them to His religion, Islâm), and has not laid upon you in religion any hardship,[] it is the religion of your father Ibrahim (Abraham) (Islâmic Monotheism). It is He (Allâh) Who has named you Muslims both before and in this (the Qur'ân), that the Messenger (Muhammad SAW) may be a witness over you and you be witnesses over mankind![] So perform As*!Salât (Iqamat-as-Salât), give Zakât and hold fast to Allâh [i.e. have confidence in Allâh, and depend upon Him in all your affairs] He is your Maula (Patron, Lord, etc.), what an Excellent Maula (Patron, Lord, etc.) and what an Excellent Helper! (22. 78)
And Allah Says in the Quran:
83. Do they seek other than the religion of Allâh (the true Islâmic Monotheism worshipping none but Allâh Alone), while to Him submitted all creatures in the heavens and the earth, willingly or unwillingly. And to Him shall they all be returned.
84. Say (O Muhammad SAW): "We believe in Allâh and in what has been sent down to us, and what was sent down to Ibrâhim (Abraham), Ismâ'il (Ishmael), Ishâque (Isaac), Ya'qûb (Jacob) and Al-Asbât [the twelve sons of Ya'qûb (Jacob)] and what was given to Mûsa (Moses), 'Iesa (Jesus) and the Prophets from their Lord. We make no distinction between one another among them and to Him (Allâh) we have submitted (in Islâm)."
85. And whoever seeks a religion other than Islâm, it will never be accepted of him, and in the Hereafter he will be one of the losers
(Surah 3:83 - 85)
As Muslims we practise our Religion as Allah intended. A Muslim's life should be seen as an example of good character, kindness and fairness.
We tell people about Islam, because we wish they too can share in the message of Islam. It is from Allah (the most High), he revealed this message to mankind many times through the Messengers he sent to mankind.
Islam's message is to worship Allah alone, without associating any partners to him.
By believing in this message of Islam, people will know their creator, they will know they were created with a great purpose in this life (to worship Allah alone), they will know how their life aught to be and how they will be rewarded. For the ultimate reward for the people who are Muslims is to see the face of Allah. The reward of Jannah (paradise) is not merely one of physical delights, but also more importantly spiritual happiness. Where the people will feel at ease, with no worries, no pressures or fatigue. They will understand and have knowledge of their lives and their accomplishments. They will feel satisfied and will glorify Allah day and night. THey will fall prostrate to Allah and will be drawn near to their Lord.
There is no greater gift for any human being other than Islam. It is a gift from Allah, it is a gift of mercy and love. Islam is complete and perfect and it is something preserved till the day of judgement.
It would be a true shame for anyone to reject this beautiful faith after the evidences have been bought before them. They will be questioned on day of judgement about that.
If there are any problems, just PM me.
You can Learn More about Islam by reading these articles:
Fundamental Beliefs in Islam Tawheed and Aqeedah
Introductory Articles About Islam