according to atheists why was christianity invented
Like so many of the most successful businesses under the header "religion", it was created to enhance the political power of a group of people by either 1) giving masses of people hope and using the momentum of their numbers to propel the religion's leaders into the fore or 2) convincing regional governments to comply with and enforce its tenets through police/military force or both. Either way, Christianity's true incorporation as an institution - centuries after its founder and main salesman (a.k.a Paul) walked the earth, and following decades of splintering and fragmentation of multiple highly divergent Christian belief - was about political power at an imperial level, and much less about Christ himself.
if the existence of a divine being does not harm anyone why oppose it and say its nonsense?
It's what "true believers" have so often done and continue to do, with their beliefs that is the problem.
The Crusades
The Spanish Inquisition
The Salem Witch Trials
The bombing of medical facilities
The assault and murder of gays
The insertion of fiat laws like DOMA
The insertion of fiat laws requiring the teaching of demonstrable lies to our kids.
You are entitled to determine Your beliefs, you are Not "entitled to determine Everyone else's beliefs.
As an atheist, there's not a single thing any religious person can tell me that'd somehow offend me. They're free to expand their fog of ignorance to include what they think of me, and that's just sad but not offensive.
under what circumstances should you convert religions for anyone?
None. And neither should you. We should not cease being honest about our beliefs (and pretend to be those of a partner) otherwise there's something incredibly wrong with the relationship.