Christianity started as a sect within Judaism.
Islam started as a sect within Judeo-Christians group as well.
The group forming within human is a continuing evolutionary process.
It is based on idea / ideology (religion is merely a form of ideology) which forms a shared identity.
Other than religion, the idea / ideology can also took the form of nationality, ethnicity or merely a shared short term goal.
That's the nature of human.... and of course as God has designed them to be. The real God that is.
Hoping that human didn't act like the above (Such as there is no sect in religion x) is a futile hope because it's not based on reality, it's not based on what God has designed human to be.
What human need to learn is how to work with those differences by accepting that there were, are and always will be differences and try to avoid brutal conflict / killing of each others.
The group formation is very dynamic and can quickly change over time due to different circumstances and (shared) goal. The 'enemies' / 'rivals' of today can be 'allies' / 'partner' of tomorrow.
Example:
The Brits and the french were enemies fighting brutal war over the span of 100 years. And become close friend, allies and partner in a very short time due to common threat from Germany.
(Note: Brits, French, Germany are all a sub-group/sect of once shared identity of Ancient Germanic tribes named Anglo/Saxon, Franks, Germanic tribes united in their effort to rebel against Roman empire colonialism, over time they evolve differently the language evolve differently, the culture/tradition evolve differently, but traceability can still clearly be seen)
The cohesiveness of the group named "The Brits" is shaken due to disagreement whether to stay in EU or leave. The shared identity of "The Brits" might be dissolved to form a sub-group (a.k.a sect) over shared idea of Pro-EU and Anti-EU.
And millions of other example across the globe throughout the history..