Islam was completed with the commandments of Allah (SWT) through Muhammad, and came to a close on the day of ghadeer khum(Q), in the life time of the Prophet (S).
What people did without the permission of the Prophet Muhammad (S) was outside of the realms of islam. There was monotheists of the time (al hanafiyyah), but also lots of pagan idol worshippers. Traditions states that the monotheists built the kaaba before muhammads lifetime, but the pagans paid money to have their idols put inside. And finally, a pagan became the ruler and he marched inside ceremonially (i forget his name) and put his idol there. This brought conspiracy and with it, warfare. Muhammad (S) tried to join the believers in Allah (monotheists) but they were corrupted, similarly when Jesus Al Messiah (S) sees in the temple and overturns the tables and says u make my fathers house a den of merchants. Similarly, Sayyidinal Muhammad (S) sees in the kaaba, idols allowed to be in the kaaba (god’s house/baytullah) because of money, instead of God’s decree. It is his mission to overturn these tables. Should he (S) not emulate the message of Jesus (S)? Firstly, it is his right to spread the message of rejecting idolatry in all its forms. The government which was corrupted, began losing money, because the different pagan tribe leaders made money by peolpe giving large donations for the blessings of the various idols.Secondly, it is his right, as the Prophet of God, to destroy these idols, to overtake power over mecca, and give that power back to the original builder of that city, the rightful sultan, Allah (GOD/Monotheistic Rulership). Now, when this happened Muhammad and his followers were cordially asked to leave mecca and not come back, on pain of death. So, the Prophet of God is now asked to abandon God’s house on earth. What would you do? Well, Muhammad (S) left peacefully, planning out his next move. His plans consisted of sending his best intellectual islamic debaters on white horses carrying scrolls to various rulers inviting them to Islam and Peace. But, if they did not accept islam, they still were offered peace. But, if they refused peace, they were labeled as enemies but not attacked. Muhammad never went conquering nations, read the history. After Muhammad’s death, chaos ensued, hippocrites had infiltrated every level of muhammad’s (S) life other than his blood relations and those who he proclaimed as his children. They were not all hippocrite companions, there was very many great companions, but some of the closest confidants of Muhammad (S) were hippocrites, mushriks, and disbelievers. Similarly, for you Jesus (S) had to deal with Judas, the mushrik, who betrayed him to his death for a petty sum of 30 silver coins. And there was also the jewish priests who were also hippocrites, and they challenged Jesus (S) to debates, for they feared he may be the Messiah (and thus lose money).Muhammad (S) also, acted this way towards the hippocrites as Jesus (S) to Judas and the priests. They debated with him (S) and said show us a sign if you are the messenger, and Muhammad (S) by Allah’s permission pointed to the moon which then split in half by his pointing, and they denied his (S) sign saying “this is clearly black magick.” Meanwhile the Meccans were planning to attack the muslims in the desert lead by Abu Sufyan (allahs curse be upon him). They marched out and found the muslims and a battle ensued while the muslims struggled to defend themselves and their prophet. For they threatened to extinguish the flame of islam (goodness), the meccans lost the battle to the muslims.Now there was a jewish group of militants in mecca, and they had common folk but mostly army men. They signed a peace treaty with the muslims that their fortress was not going to do anything violent, but they broke the treaty and the muslims went there and asked them why, and they cursed and spit and jewish guard beheaded a muslim and the muslims yelled and attacked the jewish fortress and its caravans.Then finally Muhammad goes to hijrah to medina and the children are singing welcome song to muhammad tala al badru alayna with palm fronds as he rides into medina on a donkey, (like jesus in jerusalem). and thats how muslims got medina, by love. next, they recieve intelligence the meccans are back and oh **** theyre angry and gonna attack medina. they dug a huge trench because they were wide open. And muhammad said fight them and slaughter them until they are no more and so they did. But it doesnt mean to kill disbelievers for all time. Other fights/battles were caused by a woman named hamd, abu sufyans wife, because she wanted to eat Hamza’s (a close companion) heart. And that’s pretty much the story in a nutshell. Muhammad never told them to conquer nations, period. He said “bring the Message to them and offer them peace, human beings are equal like the teeth of the comb are equal.”
Holy Quran 4:90
“Except those who join a group, between you and whom there is a treaty (of peace), or those who approach you with their breasts restraining from fighting you as well as fighting their own people. Had Allah willed, indeed He would have given them power over you, and they would have fought you. So if they withdraw from you, and fight not against you, and offer you peace, then Allah has opened no way for you against them.”

As muslims we must believe what quran tells us. And according to quran, you and I are all going to heaven as long as we follow the authentic teachings of a prophet of god.
Holy Quran 2:62
“Verily! Those who believe and those who are Jews and christians, and Sabians, whoever believes in Allah and the Last Day and does righteous good deeds shall have their reward with their Lord, on them shall be no fear, nor shall they grieve.”