Salaam o 'alykum warahmatulahi wabarakatuh.
sis, the religion of islam has started since the beginning of mankind - since Aadam (r.a). as time has passed by, people have been going astray from the right path and Allaah subhanahu wa ta'aala has kept sending messengers to put people back to the right path again.
when Ibrahim (r.a) [a forefather of our beloved Prophet Muhammad sal Allaahu alayhi wasalam] was on the earth, he got the same message from Allaah subhanahu wa ta'aala, he had the same duties as us muslims do nowadays - this includes; salaah, ramadhan, zakat, hajj.
the arabs believed ibrahim (r.a) existed too, they even believed they were a prophet (he was the arabs ancestor aswell.) the arabs also believed in Allaah subhanahu wa ta'aala, but they wanted to worship the idols even though they believed in Allaah subhanahu wa ta'aala, because they said that we cant focus on Allaah - so we use statues to worship him. they were doing shirk because they started worshiping the idols and our beloved Prophet Muhammad sal Allaahu alayhi wasalam came to re-bring the message of ibrahim (r.a).
our beloved Prophet Muhammad sal Allaahu alayhi wasalam came back to teach the arabs that there is nothing worthy of worship except Allaah subhanahu wa ta'aala. the same pillars applied, including; salaah, ramadhan, hajj. and this was the exact same religion hadhrat ibrahim (r.a) came to teach the people.
Say: Allah has spoken the truth, therefore follow the religion of Ibrahim, the upright one; and he was not one of the polytheists. (3:95)
obviously this is just a basic description of what happened, and if i can find some links to give you some more info. i will look around insha Allaah.
Allaah u a'lam. (Allaah subhanahu wa ta'aala knows best.)
wa Salaam o 'alykum warahmatulahi wabarakatuh/