God started Judaism and it devolved into chaos. There was no rhyme or reason, so everybody worshiped as they saw fit or understood. Eventually, Jesus started Christianity and it devolved into Catholicism. There was no rhyme or reason, but it was strictly enforced for about a thousand years. The majority did as they were told and probably really didn't understand or care about most of it. The Reformation started as the Catholic Church lost power and several people attempted to reboot Christianity as they saw fit or understood, based on their pet peeves with the Church.
One one hand, the Catholic Church is Christianity degraded, as it was the only representation - regardless of how false - for a long time. On the other hand, rejecting their beliefs and starting a new church being "an offshoot"? Imagine you were born to parents who told you your whole life that 2+2=3 and that's all you ever heard. Eventually, you become an adult, move out, then decide 2+2 is absolutely not 3, but 4 (or 5, or 17, or apple; whatever). Is your new belief system an offshoot of theirs or is it a rejection?
Just because the Catholic Church usurped authority over Christianity and centralized power of all churches first, does that validate their claim of possession? They reject the vast majority of the very book they compiled, yet claim to be the original and only true representative of it. I can claim I'm the queen of France and call anyone who disagrees an enemy of France and (if they're French) a rebel, but that doesn't make it so.
That being said, JWs and most church denominations cling to Catholic beliefs which were invented hundreds of years after the founder and charter members were dead. These obvious tethers to Catholic inventions give the Catholic Church some degree of evidence for their claims of origin.