Bismillah, Salam Emma, how are you? You have asked a very interesting question. If you have siblings, you are brought in almost same environment, yet all of us are not the same, rather each one is different. Islam was taught by prophet Muhammad SAW in a very pleasing way. Allah praises him for his manners too.
Now when the force comes in then the diffences start. You will find even in the companions like Umar, and Ali also that they understood Islam their way. Look at Hazrat Umar, he drew a sword after the death of the prophet that if anybody calls him dead, he wanted to cut his neck. When Abu bakar came, however, of course he had a better understanding of Islam or a tact of dealing with matters wisely. He recited a verse of Holy Quran to people that, " Muhmmmad was only a prophet of Allah, all (prophets) before him are gone too." So the people could understand and Hazrat umar cried, yet believed that his master war really deal. So you see the first historical difference. Now a very valid point is if by that verse recited by Abubakar all prophets have been dead, where is the Isa who the main stream Muslims still think about being alive on skies. So the answer to that is that he was dead too. Otherwise the companions must have stood up and claimed that Isa is dead so why can't our prophet be alive. This needs to tell you another important fact. Today there are 4 kinds of beliefs about Isa AS. Jews think they were successful in killing him a cursed death see Deut 21. Christains assume that he died, yet resurrected from the grave after 3 days; this is true, yet they want to believe in the cover up; that Isa did go away from that area to find the lost sheep of Israel ( the other 10 tribes). Yet Christains want to believe also that is is sitting on skis on the right side of God and will arrive in the later days; which are these days by the way. Muslims generally believe that Isa was a prophet of Allah so another person of his face was on the cross, while Allah took him to the skies. There is a 4th explaination; that he was took off the cross alive, while he was in a coma on the cross due to the shock of nails in his hands/legs. Bible does tell his bones were not broken. It was the way Allah wanted to save him to enable him to be walking soon. So what heppened that he did remain in his deciples for a few days afterwards he migrated to guide his other Jewish tribes who had already spread in the east. He travelled throuhg Syria, Iraq, Iran, Afghanistan,and aultimately setteled in Kashmir.Holy Quran tell about this refuge too with a place names Rabwah; an elevated one. So he lived long life and was burried in Sirnagar, Kashmir (Yuz asif's grave in arae o Khanyar).
Allah knew that there shall be Firqa (sects)in Islam. He told the prophet too so he defined according to the knowledge given by Allah that there shall be 72 sects in my followers (all hell bound), and there shall be a 73rd Jamaat, the only heaven bound. When asked how people shall recognize the right one. He gave a simple reply, they shall be the ones following my example. So Emma now you have a job too. Search which one is the right. You see all are declaring each and all other to be Kafir (non believer due them not being from their group).
Can you tell me what happened in the world when the 72 gathered together and declared the one "Hell
Bound"? It happened a few decades ago. Wassalam, Mansoor