Some non-Christians don't believe Jesus existed. The only accounts we have of him are slightly biased. But overall there is more proof for the existence of Jesus than for many other historical figures. Check out the FAQ for many great answers to your question.

For the BC/AD divide question I think the easiest way to understand it is a snide comment often made by modern atheists.

How can it be Thursday if you don't believe in Thor?

Not trying to be rude. The question just shows that naming conventions and standards and beliefs don't have to align.

Internationally there are a number of different calendar systems that are used quite broadly (although these may be based on other religious traditions), though businesses in these countries will also use the western calendar when conducting business with the west. (When I lived in Nepal we had calendars that enabled conversion between the Hindu, Buddhist and Western calendars).

The other Abrahamic religions (who also have their own calendars) typically acknowledge the existence of Jesus, but he plays less of a role. For the atheists whether Jesus Christ was an actual historical figure is debatable.

But it's the calendar we use so the switching costs are really large - and there's not an objectively better point for a year zero (unlike with some other things we stick with like keyboard layouts). Some people use BCE and CE (Before Common Era and Common Era) which just replace BC/AD.

But as with so many other things in western culture (that are derived from Christianity) it's part of the culture, so you just kind of go along with it.

I will make the note here that there is a consensus among academic historians that the person we know as Jesus did exist. The FAQ in the sidebar has an entire section devoted to the topic.