What are the fundamental beliefs of Ahmadis?

I need an answer. If I do not believe in MGA being the promised messiah, do I still remain a Muslim? If believing in MGA as being the promised messiah is an essential of faith and I rebel this notion, it should make me a non-muslim, right? Specially, going through the website, it is abundantly clear that this 'essential' is one of the most important one for a 'muslim'.

There is no such thing as 'incomplete Muslim'.